How I was going to actually turn the buttons on the outside into digital inputs, took a bit of thinking. For awhile I had the idea of 3D printed inserts and a brace of sorts with places for buttons. I had also planned on making it one connected structure that would fit around all of the nooks and crannies.

Instead I salvaged the plastic inserts from the cassette guts, since these would be better than any inserts I could create. Also instead of one solid structure I went with a more modular approach, creating a small piece that could be printed in quadruplicate.

The tactile button sits against that back rail and the insert sits on the front and is lined up with the button.

And with the help of some cardboard and hot glue, all the buttons sit nicely up against the inserts and function as intended.