So basically you want to have a coin door with no mechanism, where you drop the coin through the slot, have it hit against something to set off a switch, then fall away into oblivion?
I'd suggest one of the following:
1) Don't have a coin door at all, just use a button to "coin up", or
2) Check out eBay. There seem to be a lot of used coin doors with mechanisms there, many of which are fairly cheap. Alternately, you might post on one of the rec.games.video.arcade newsgroups (and/or the trading board here) - someone's bound to have a coin door with mechanisms they'll sell you cheap.