Have you seen any pictures of my Space Firebird? It has a factory wooden door with reject buttons on it (the coin slots themselves are on the panel), maybe that would be an easier solution.
Also, you know you can just rig a real coin door to do the same thing, right? I have several machines that credit up by activating the coin reject buttons.
Another option might be to snag a cocktail coin acceptor plate (with reject switched) and mount that on a small wooden door.
But like other posters said, they already make small coin doors, and a used one (once repainted) would work fine, and almost certainly be cheaper and more authentic than one you would make yourself.