I did the same sort of thing as dan-o but instead of using burlap I'd suggest using netting. Burlap is a PAIN to haul up and is slow to bring up because of the water resistance. My "stage 1" trap was made of burlap

Youre going to want about 2.5 feet of hang to the netting. Run a wire across the rim from one side to another, and throw a bait box in the middle of that. It will be a better option than wiring it up to the bottom of your trap as if you put it on the bottom, it may end up drifting under the rim and making the bait inaccessible to the crabs, if that makes sense.
Bike rim works great though and you can easily drill through to tie your four ropes to. I ran them all through a ring before tying them together in a knot , seemed to help. You can basically just sew the netting together using bailing twine.
I found that my trap worked best if i left it down for a maximum of ten minutes per haul. This keeps crabs from getting their fill and leaving..
Good luck, I had great success with my homemade trap...it's always more rewarding when you do well on homemade gear !