Perhaps make a box with a couple thin metal straps coming out from under the lock bar using the lock bar to hold them in place. Or mounting the box with coin door screws so when removed you have done no long term damage to your machine. Then use some Velcro to make a more solid feel if necessary.. Remember to round off the sharp corners & edges on the wooden box a little to make things a little safer.

With the wooden box idea you could even add buttons to both the ends of the box & top of the box having two different ways to control flippers.