I got my signs at the surplus pile at the state highway engineer's yard. Aluminum signs are very hard to recycle because the coating on the signs is expensive and difficult to get off, therefore making them unprofitable to melt down. Old signs are sometimes just thrown in a big pile and forgotten about . . . all you need to do is ask. I found three the same size, 15" x 21".
Don't steal signs. They are expensive, paid for with your tax money, and you could potentially create a dangerous situation by removing an important sign from the street.
All the wood was found 2" x 4" scrap that I milled down to about 1-1/4" square. The only expense was the screws, 3" drywall or similar, that ran me about two bucks for half a pound.
You will need these materials:
Approx. 16' of wood per stool
3" drywall or wood screws
You will need these tools:
Miter, circular, or jigsaw
Impact driver and/or drill
Step 1: Framin'
To make the top of the seat square, take the short dimension of the sign, in this case 15", and make that the length of the top piece of your "A" frame.
Cut the legs to your desired length with a 5 degree miter to get a nice taper effect. A rule of thumb is that a stool should be a around 8 inches shorter than the surface where you will be sitting. So, for a 42" bar, make your stool 36" tall; for mine, designed to match up with a standard 36" kitchen counter, I made them 28".
Make the miters at each end of the legs parallel to one another so the stool will sit flat. Measure up from the floor 6-10 inches for the crossbar brace.
Attach everything with one long screw and a healthy dose of glue. Pre-drill for your screws with an 1/8" bit to prevent splitting, and counter-sink your screws so the heads are flush.
Knock down all the sharp edges with some sandpaper.
Step 2: Signin'
Begin by measuring the long side, in this case 21". This makes the sign 6" longer than it is wide. To make a square, 15" x 15" seat, measure in 3" inches from each end and strike a line across.
Space your holes at 1" on center on each line. Drill an 1/8" pilot hole, then go back, alternating 1/4" and 3/8" holes. The alternation isn't necessary, but I did it for visual interest. Feel free to just do all the same size holes.
While you're at it, punch the holes for the screws to attach the sign to the frame, three along the edge of what will eventually be the seat top, and two on each side of the turndown.
Step 3: Bendin'
Step 4: Assembly
Pop in a cross-bar with two long screws per side and some glue.
Take a rounded bastard file and hit each of the holes along the seams so they won't scratch or snag clothing. A couple quick strokes per will do. Also make sure to sand or file the coating on the sign where it cracks; the sharp plastic there is actually more of a scratch hazard than the aluminum itself. Older painted signs don't have this problem.
Clean them off with some dish soap and a sponge, then a final swipe with Windex and have yourself a well-deserved seat.