I think the coin buttons need a plate behind them.
Either metal or ABS plastic.
If it were me, I'd make a fake coin door where the speaker hole currently is and then mount the speaker elsewhere.
I'd do small metal plates on top a large black plate to mimic the original donkey kong coin door style.
It doesn't have to be a exact replica. I think your buttons mounted on top of metal rectangles mounted on top of a black square would look good.
EDIT: check out metal access panels at your local hardware store. They aren't that expensive and would make a convincing coin door that actually opens.

Here is a fake 1-slot setup I made.
The coin return is actually a gutted square button with the top lens painted black and glued in backwards.

Not perfect, but my whole shtick with that cab was using what I had laying around and not spending any money.