Plyometric boxes are a fantastic workout tool and are common sight in Crossfit Gyms. They're quite expensive to purchase but much cheaper to make, and they aren't overly difficult to construct.
At my gym we've got a number of different boxes, several each of 12", 18", 20" and 24" heights. One of the other members told me that she wanted to progress past the 12" box she was using, but wasn't quite ready to move to the 18" box, and asked if I could make her a 15" to bridge the gap. For that reason these instructions will be based on a 15" box, but once you understand the basic construction you can easily make plans for any size that you'd like.
I'm also going to go into depth regarding setting up a workflow that lends itself to churning out multiple units. The woman that asked for this only wanted one, but as soon as I started working through the layout, I realized I could get four boxes out of a single 4' x 8' sheet of plywood. (Almost. I had some plywood laying around that I used to make two of the tops. Everything else came out of one sheet.)
The idea, though, is that if you're going to go through the trouble of making one, you might as well make a bunch, because it's only a little more effort. A lot of the work involves setting up jigs and templates, but once they're done you can just keep cranking them out.
I should also note that I have a pretty wide array of power tools. In most cases these just make things easier or faster, but many of them aren't a necessity. As I work through the instructions I'll try and point out how to adjust things if you don't have everything I do.
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Signing UpStep 1: Tools & Materials
Tools
Circular Saw & Guide - The first step will be to cut a 4' x 8' panel down into all the pieces you'll need to assemble your boxes. The more accurate your cuts, the better your box is going to fit together. At the bare minimum you'll need a good 8' straightedge to guide your saw. I use a guide system which consists of a pair of 50" aluminum extrusions that mate with a plastic base I've got attached to the base of my saw. A channel in the base rides in a raised portion of the guide so it tracks a straight line.
Drill/Countersink - I drilled and countersunk my holes with my cordless drill and then used a smaller pocket driver for my screws. A countersink is a pretty cheap investment to ward off splitting the ends of your plywood. Spend a few bucks to pick up one of these if you don't have one already.
Layout tools - At the very least you'll need a measuring tape. A decent square is handy as well.
Optional
Clamps - I'd almost call these necessary. I've got half a dozen quick clamps and I think I've used them in every single project I've ever worked on. They're almost like having a second set of hands. I could probably find a way to get along without them but it would be a lot harder.
Jigsaw - It helps to have handholds in your boxes to make them easier to move them around. You might get by drilling a couple of holes with a hole saw, but oval handholds are a little nicer.
Router - In my setup I made a template for my handholds, used the jigsaw to rough cut the holes and then used a pattern bit on the router to match them to the template. A faster way to work, but you could just use the jigsaw to cut the holes out.
Biscuit Joiner - A glued butt joint with screws is a pretty typical way to build these boxes, but biscuits add strength as well as keeping the sides perfectly aligned while you're assembling the boxes. If you have one, I'd use it.
Pocket Hole Jig - I used this for the tops so the screws would be hidden. In retrospect, I should have used these to join the sides as well, because then you wouldn't see any of the fasteners. Pocket screws actually make a stronger joint, because you're screwing into the face of the plywood instead of the end grain. If I make any more of these I'll go that route.
Orbital/Belt Sander - Now we're just getting into cosmetics. The boxes would be fine without these, but I'm anal retentive so I couldn't skip the finishing step.
Materials
Plywood - This is going to be your main cash outlay for the project, and the quality of the resulting box will depend on it. Hardwood plywood will get the best results. Particle board, MDF, OSB or any other sort of sheet good maybe be tempting but would be bad choices for this type of application. My local big box store had 4' x 8' sheets of maple ply for only $36, which is a steal. Shop around and you'll probably find a good deal.
Screws - Lots of people like to use drywall screws, but I prefer to use decking screws. I like Torx drive screws, and I've found the decking screws to be a little hardier than standard drywall screws. (Occasionally I'll snap the neck off one of those.) Personal preference, though. Use whatever you like, or already have on hand. I went with a 2" length.
Pocket Screws - If you've got a pocket hole jig you've probably already got these sitting around. Mine are 1 1/4" coarse thread Kreg screws designed for pocket holes.
Glue - Use a glue designed for use with wood. I like Titebond II.
Biscuits - If you're using a biscuit joiner, you'll need biscuits. I used size 20.
Polyurethane - I used this for a finish on the sides of the boxes. You could leave them unfinished, or put on whatever you'd like. The other ones at my gym were painted, but I opted to just stick with a clear coat. I had a few cans of spar urethane sitting around, so I went with that.
Top Material - You'll probably want something on the top other than smooth, bare plywood. There was some leftover textured paint from the construction of the others at my gym that I used. It's paint with an epoxy additive for wear resistance and a silica sand suspension for texture. When it's dried it's basically like sandpaper. The ones at the gym have seen a couple of years of constant use and they're still going strong, so this is a good choice. You could also get some rubber floor mats and cut those to size for the top and cement them on. If you go that route, make sure you calculate that into your height equation when you're laying out the boxes.
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just need to find a cheap source of ply or make a stupidly heavy one out of scaffolding planks.
I would suggest, though, that if you're going to make another box, don't make one of these. The latest ones I made for the gym were a totally different design (swiped from Rogue Fitness) that I think gives you more bang for your buck.
Basically what you do it make a closed-side box such that each dimension gives you a different box height. I made 3 copies of the ones they use at the Crossfit Games (20", 24" and 30") and I also made two that were 18", 20" and 24".
If you're going through the trouble of making one with the angled sides, this isn't any more complex, and it'll give you 3 more heights to play with instead of 1. I was worried that they'd be a little tippy while using the highest heights, but they're really heavy and I use ours all the time with no problems. They're solid as a rock.
I have no idea how Rogue makes theirs, but I tried to figure out how to brace it internally to deal with the stresses. You can see in my pics how I did it. Not sure if that's the best way, but I haven't seen any trace of them breaking down yet. Let me know if you have any further questions.
So let's assume you're going to make a 20" x 24" x 30" box.
First you'll need a top and bottom. With mine I chose to make these the 20" x 30" pieces. (I think I did it this way because when I was trying to figure out how to cut everything out of a sheet this was the most economical.)
If you're sure of your ability to cut things perfectly then you can make these to size. I cut mine maybe 3/16" over so I had a little overhang. When everything was assembled, I took a flush cutting bit on the router and trimmed them, running the bearing on the sides of the box. Otherwise they probably wouldn't have mated up perfectly.
So now you're going to cut two more sides. These will be full length (30") but have to be a 1.5" shorter to account for being sandwiched between the top and bottom. So they're 22.5" x 30".
You could make a hollow box shape now by setting the top and bottom on the edge pieces you just cut. It'd be 24" tall (22.5" plus .75" for each top and bottom) and 30" long.
Now you need some side pieces, plus one more for internal bracing. This FACE will be 20" x 24", but that includes all four edges you need to account for. So your piece needs to be 18.5" x 22.5".
I also added two more pieces to complete the cross section you see in the last picture above. These are tougher to measure perfectly ahead of time. 3/4" plywood isn't exactly 3/4", it's like 22/32". So the box will be a hair off exact dimension when finished. But if you try to precut those last two supports they'll be a little loose. So wait until you dry fit everything, then measure and cut those pieces.
You might be able to get away without them. I would guess the Rogue boxes don't use them. (The next time I see one I'll peek through the handhold and see.) But I asked myself how the box would hold up if it were being jumped on.
At the 24" height, the platform you're jumping on is supported by all four sides as well as the crosspiece in the center. Bombproof. You could probably set a car on that. At the 20" height, you're directly supported by the edges of the three matching internal pieces as well as whatever mechanical holding devices are keeping the top and bottom on. (Screws, nails, glue, etc.) Still pretty solid. The ends were what I was worried about.
Without those last 2 internal pieces, the ends are being held in place entirely by mechanical fasteners. There are no edge-on pieces of plywood to help transfer the forces down to the ground. So I put those last 2 pieces in there to do just that.
It might have been fine without, but I don't want something I built falling apart when someone is using it.
I used pocket screws on my box on pretty much everything I could reach. If you do that, think ahead to how big your drill/driver is and how much space there will be once things start going together. I have a pretty small Bosch driver, and it was still too big for some of the tight spots. I hadn't thought it through and might have gotten around that if I could have put the entire internal cross section together first and then slid it into the sides. If you do this, you'll still end up with at least one piece, probably the top or the bottom, that you obviously won't be able to use pocket screws on. Unless you have a really small monkey that can operate the driver and crawl out through the handhold when he's done. I just nailed the last side on. And use plenty of glue. Glue + mechanical fasteners = good.
When these are done I think they're close to if not over a full sheet of plywood, so they're pretty darn heavy. They're far more stable than I would have thought.
Mike