1/2" x 2" molding to make the frame, 2" x 2" pressure treated wood for the
corner posts, 1/2 x 1/2" garden fencing for the netting, lattice caps for
the transfer door tracks, and pressed board for the transfer door.  Material
costs about $50 and takes about 1 1/2 hours to make- or less if you have
help.

I used molding for the frame because its light weight- heavy wood could hurt
a cat if the trap comes down her.  Plastic garden fencing isn't the greatest
but it works- you can use two layers on the sides if necessary.  Don't use
chicken fencing- the thin wire can seriously cut a frantic cat trying to
escape.  The square openings in garden fencing are large enough to use 3/4"
dowels or broomsticks to guide the cat through the transfer door and into
the transfer cage.

Here're some pictures of the one I just made:

http://maxshouse.com/drop_trap.htm

I'm going to make a lightweight folding trap out of PCV as soon as I get
strong cloth netting- I'm going to try fishing net if the holes are small
enough.

Phil