A friend referred me to someone who had an Happ coin door for sale. It was several years old but it was never used, and still in the original packaging from Happ. For $35 I couldn't resist. For me, a coin door on the front really helps make the cabinet look authentic!
Since the part was so old, I didn't know if there were any measurements available, so I just measured the actual part. I measured the front door and measured the correct size in order to mount the coin door exactly centered. I used a t-square to be sure that the hole was exactly straight to the top and sides. The tricky part was trying to get the curves right. Basically I just eye-balled them and they worked out just fine.

Attaching the door to the front door was pretty easy. There are 5 "fingers" that come out from the back of the coin door. These fingers can be used to mount the door on 1/2" or 3/4" plywood, in my case 1/2". The screws hold the fingers tight to the wood and presto!

On the right here is a picture of the final door from the outside. Looks pretty authentic to me.
The only think I would consider doing last would be to wire the lights for the coin slots and replace the "Happ Controls" sign on the coin release button. We'll see.