i done something similiar years ago,but had to tie wrap a heavy duty iron dumbell pole to the bottom of the door....so that it shut as fast as possible...i ended up buying one off ebay £90 delivered...brilliant bit of kit....
Ok cheers. I will try the home made. I'm a tight fecker when it comes to spending £90. Lol.
the way i done it was simple....string on a stick holding the door open...took up through the top of the cage,over the back,and back in tied to the bait.....therefor when they took the bait....and pulled it...the stick came free letting the door drop.......simples.......