1/2" square, 36" wide Wire is from home depot, and costs $15. You can make 3 traps easy out of a single small roll of wire.
Lay out wire flat so its easy to work with
Measure 3" and set a straight edge to bend the wire against.
Measure 12" and make second bend
Measure another 3" and make another bend.
Cut the wire 4" from the last bend, this extra 1" will be used to hook the edges together
Hook the 2 ends together, wrap wire ends around adjacent edges. It should resemble a box now
On each side of the open end, measure 4" and cut back along the bend, then around to remove 4" of mesh on all but one of the 12" wide sides.
Bend the 4" piece back and into the trap, forming a ramp. Wire the sides so it makes a 1.5 to 1-3/4" gap between the edge of the ramp to the top of the trap.
Repeat for other side. You can choose to close off the other side completely, but I find making a ramp on both sides allows more crawfish in the trap.
In the bottom of the trap in the center, cut out a 4x4" hole and make a hindged door to allow placement of bait.
Spray trap with a non-reflective paint. Tie a braided string to the center of the top to allow placement / recovery of the trap. Use whatever you want for a float marker (twist top pop bottle works fine).
Building these is a good fun weekend project with the kids.