Crossfit became my main exercise/fitness regimen this year 2011. Check it out here:
http://www.crossfit.com/

I setup a home gym in my garage so I can workout whenever its convenient. Cross fitters use plyo boxes, they are expensive to purchase so I made my own. I found this great website with the idea for a three in one height box:
http://michaelmneal.blogspot.com/2010/09/crossfit-plyo-boxes.html

You may not have all the tools I used, so I used various options along the way to show how to use what you have available.

My friends and family enjoy jumping on the box, and doing pullups with the assistance bands. Check out this instructable (my first) to make your own!

Remember that power tools and jumping on/off boxes carry inherent risks. Be careful, read your power tool manuals, and save the alcohol for after your building and jumping. When you are done I recommend Sierra Nevada Celebration!

Step 1: Layout Your Plans & Make the Cuts

I drew up the attached cut list to use an old 4x8 foot sheet of weathered plywood for my box. I minimized the cuts, and ensured that cuts of the same width were made together to limit the # of times I had to adjust my table saw. Follow this guide and you'll save time.

I made the first cut with my circular saw. You can make all the cuts this way if you don't have a table saw. Hold the plywood off the floor using spare 2x4's or something similar. The straightedge I use is one I made, but you could use another board. Clamp the board, and run the circular saw against the straightedge.

Step 2: Finish Cuts

I used my table saw for the cuts after the first one. I find cutting the plywood in half, makes getting it through my smaller table saw much easier than trying to manipulate a full sheet.

You can see all the cut pieces laid out as they originally fit into the 4x8 sheet. There are two scrap pieces, and two pieces leftover you can use on another project.

Step 3: Cut Hand Holds to Carry the Box

I recommend you complete this step now, although i worked on it after assembly. If you do it earlier, you can round off the edges of the cuts on the inside of the box, which makes it nicer on your fingers when you carry the box, avoiding splinters.

I used three methods to cut them, with a drill, with a rotozip, and with a router.

1. With drill
If you have only a drill, you can use a hole cutter, and high speed on the drill, to carve out the opening you want. The edges will be rough. You can use a rasp or file or chisel or sanding block or dremel tool to smooth it out.

2. With Rotozip
This produces a faster and smoother cut, although again the edges are left sharp so you need to round them with a sanding block or rasp or file.

3. With Router
Using a drill and cut a starter hole, then you can use the router with a straight fluted bit to cut the opening. Then use a bearing round bit to smooth inside and outside edges. This produces the best result, and I finished both sides off with this method.

Step 4: Assemble the Box

Screw the box together. The challenging part is holding the parts together initially. I have some nice clamps and a friend to help me dry assemble it. If you have good wood glue, you should use it in conjunction with coarse thread 2" drywall screws. Try driving the screws in without pre drilling first. If you can easily drive in the screws that will provide the strongest result.

If you don't have the clamps, you should find a helper to hold two pieces together on the edge of a workbench. The helper should hold the pieces tight against a framing square, or speed square, while you drive in a three screws through the edge.

Once you complete three screws in each edge, then add more screws. Target a screw every 6" or so along the edges.

A stronger box will result if you glue and screw the box together. That is my recommendation.

Step 5: Finish the Box

I used a belt sander to remove the weathering from the sides of my box, and to round all the edges and corners. This will reduce the likelihood of scraping skin off your shins, or pulling long splinters from under your skin. I started with a 50 grit, then 80, then 120.

If you don't have a belt sander, use a rasp and/or sanding block. Same results, just takes a bit longer.

For a nicer look you can paint the box (add some sand to the paint for "grip") or using skateboard grip tape on the box. A clear polyurethane would look nice but might get slippery in humid warm weather so don't use that. I left mine unfinished for now, as it will stay indoors until next summer.