In this thread, we covered OT(2) and DT(2)
I may have heard of some who did it to their ST, as well 
If one is used to soldering and tinkerning, it does take 5-10 Minutes. If not so experienced, I’d go for an external battery, as you could easily velcro it do the bottom of the board.
https://www.elektronauts.com/t/digitakt-modified-with-internal-battery/
I’m still tinkering a bit on the files. Usually, I’d do the screws with threaded inserts, but having pretapped holes should also to the trick. Will check out.
Yes, more or less like that. Material recommendation: If you only plan to use it inhouse, use PLA. Top for that usecase, but absolutely hates UV light. You can use PETG in that case, but it has a little more of a flex in it, but that does not really matter in this construction. ASA would be perfect, but should and mostly cannot be printed at home bc nasty fumes.