This structure allows you to set up a slackline without using trees, poles, or A-frames. It's very easy to build!

If you enjoy this project, then check out my books: Rubber Band Engineer and Duct Tape Engineer. || More engineering projects || Everything I make

I suggest reading through this Instructable for ideas and then making improvements wherever you see fit. I'm definitely not a pro :)

Step 1: The Nuts and Bolts

Please report broken links.

There's not much to it. If you want to copy my design, you'll need:
8'-foot 2x4s (x9)
6" long ½" bolts (x12) with nuts (x12) and washers (x24)
Simpson Strongtie End Post Cap LCE4 (x4)
Simpson Strongtie Tie Plate (x8)
Simpson Strongtie Shelf Bracket CF-R (x2)
Nails for Strongties
3" Wood screws
A hammer, drill, and saw.

Step 2: Build the End Posts

For both end posts, I used:

4ft-pieces of lumber (x2)
2ft pieces with 45-degree angled ends (x6)
3ft pieces (x4)

In retrospect, the 3ft pieces should have been at least 6ft.

Take a look at the photos to see how it all fits together.

Step 3: Drill Holes and Assemble

I think the easiest way to drill the holes is to line up your lumber and clamp it together, then drill through multiple 2x4s at one time. Even if your measurements aren't perfect or the holes are askew, at least the bolt will fit through smoothly, which is what's important. I discovered this technique halfway through construction, so some of my beams did not line up perfectly.

When you're finished drilling, keep the clamps in place and insert the bolts. Be sure to use washers on both ends of the bolt. I used a mallet to pound the bolts in, which helped speed up the process.

If you need to take the bolts out, don't make my mistake of hammering them out from the threaded end - I ended up ruining several bolts by hammering the threads away, making it impossible to screw the nut on.

View the photos to see where I drilled the holes.

Step 4: Setup Your Slackline

You'll need about 40' of webbing to setup a slackline on the structure.

Here's how I set up my line with just 3 carabiners and an O-ring:

When the slackline is setup and tighened, the whole structure bends into a shallow 'U' shape, which causes the end posts lift off of the ground by a few inches. I solved this by simply elevating the end posts with cinderblocks.

I've never seen this setup used by any professional, so I can't guarantee that it's safe, though it has worked well for me.

Step 5: Improvement Ideas and Tips

I suppose now is a good time to confess that I have never built anything out of lumber except a few woodshop projects in high school. But I do teach engineering to kids for a living, so I figured I could apply my trade to this project. This means that I built this the way I structure my lesson plans: build first, test later, and then redesign til it works. What you see in this Instructable is the result of an initial idea transformed through testing and changing things until I felt the rig was solid. I also lacked some crucial tools (which is why I entered this into the wood contest... hint hint). So naturally there's still room for improvement.

Improvement ideas

Reduce the cost of the project by finding a more efficient design. The Strongties ate up a lot of my budget! Buying many bolts is also costly, so using them more efficiently is worth thinking about. Salvaging or buying bargain lumber for the non-load-bearing elements will reduce the cost even further.
You could try using three more 2x4s to make the structure 8' longer. I think the structure is strong enough to support a longer line, but the end posts will lift up off of the ground more. This could be solved simply by adding bigger feet to the end posts or weighing it down with sandbags.
Make the end posts taller. When you're not using the slackline, it could be used as a clotheline, or you could hang a hammock from it.
Paint the wood with a heavy duty exterior paint or outdoor finish.

Tips

I undo the line at the end of each day after I'm finished slacklining. I believe this will increase the structure's longevity, and you'll also become a pro at setting up slacklines. The anchors and linelocker don't need to be undone each time the line is setup.
Protect the structure from weather. Mine is kept in an area that receives only a few hours of direct sun, and when it rains I throw a couple of tarps over the whole thing.
Don't move the structure while the line is under tension.
If you're a beginner, learn to slackline over sand, grass, or padding. A beam of lumber and bolts isn't ideal :P

Have fun!