Ok,I'll explain as best I can without pics.for a 12x12x36 trap you will need:
5'of 2x4
3'x6' heavy chicken wire,(fox pen wire)
12"x12&1/2" 3/8 plywood
12x12&1/2 sheet metal
3"nails and 1"staples
i'll give the measurements assuming you are using rough 2x4 that is a true 2"x4" instead of finished which is smaller.
Cut a length of 2x4 48" long.Mark the center and chalk the length,using skilsaw set to 1/2",rip a channel the length of 2x4.Make several passes until the channel is 1/2" wide x 1/2"deep x 48 long.It should be off-center,that is there will be 2" on one side of channel,1 1/2" on other.(a router would work better for this if you have one).
Cut the 2x4 into 3 pcs,2x18 and 1 @ 12".Nail the 3 pcs together forming a U-shape with the channel alligned inside.next,nail in the 4th pc,2x4x12".This piece will be 10" from the bottom of the "U",and turned so as not to obstruct the door channel.Drill a 1/4" hole in the center of the fourth piece(this will be the trigger).this completes the framework of the door.Cut a plywood door to fit,cover one side with sheet metal.Form the cage part of the trap and fasten to frame with staples.Trigger assembly is nothing more than a length of mechanics wire and 3" nail.The nail goes through the 1/4" hole you drilled to hold the door up,dangle the other end through the cage near the rear and hang a can of sardines/other bait.Set the nail "hair triggered".
The trap is very lite and needs to staked to the ground so coon can't tip it over and escape once caught,as gravity is all that holds the door down.Sheet metal will prevent coon from clawing/chewing the door and getting out.
hope this is helpful.Not sure how clear this will be without pics.Just imagine a guillotine?Good luck