WNC: I built several cage traps when I was back in 'America'. For cats, bigger is better. Most of mine were something like 24x24x48. I haven't built one in a while and don't have any pictures, so I will tell ya what I know as best I can-which may be pretty poorly!
I like expanded metal for the cage. You don't need a frame and coons don't hurt it. 1x1 inch wire works fine, but coons occasionally tear it up and it gets mangled over the years.
I always used old road signs for the door. The county can't give em away, but they can sell them. I paid a buck each for about a dozen. The door slides vertically. I mounted it in a track made of thin pipe (1/2" conduit) with a slit in it. Saw the slit with a metal blade and a table saw. Make sure there is something at the top and bottom (a piece of angle iron works well) so they can't bow the door out--the door must be supported from all sides.
I always made the 'pan' out of wood-I think they will step on wood more readily than metal. Make an angle iron frame and put a piece of plywood in it. Coons eat the wooden pan, but scrap plywood was never a problem to get. The pan is hinged along the front. The hinge, which turns with the pan, makes a 90 degree bend just outside the trap and heads towards the back. This arm is about 6" long. Attached to it with a swivel (oversize hole with a bolt through both) is another rod going straight up to the top of the trap and supported by (running through) a couple large (1") washers welded to the side of the trap. On top of the trap there is a long rod going from BARELY under the door to the end of this vertical rod. This rod is pushed (or pulled) towards the rear of the trap by a spring. The door is lifted, the horizontal rod is put just under the edge and the vertical rod is put behind the horizontal rod (pulling the pan up). When the pan goes down, the vertical rod drops (from behind the hotizontal rod), the horizontal rod is pushed towards the back of the trap and from under the door by the spring, and the door drops.
Nothing should fit too well--leave lots of slop to compensate for rust, mud, and dents. I never bothered to paint them (cats don't seem to mind "BRIDGE OUT" or "SLOW CHILDREN AT PLAY" signs-maybe they can't read). I do always kick a little dirt and grass inside. Piling brush and such around the outside is optional--keeps people from shooting/stealing cats, doesn't seem to help with catching cats much.
Damn, that sounds like something Wile E. Coyote would write! I could build one faster than I can type that--it isn't quite as complex as it sounds. It is a bit more work than you could get by with, but these big heavy traps will last forever. I still have a few on the farm that Dad and I built twenty-some odd years ago. Mom occasionally uses them to get rid of assorted vermin (they are great skunk traps--set em beside the pond, then push trap, skunk, and all in the water for a week or so).