I wanted to create a switch that would utilize the unused screw ports on the Gameboy DMG case, as to minimize modifications, (and regrets?). This is what I came up with, and it works great! Uses 4-pin DIP 6x6x5mm tactile switches, (like these: https://www.amazon.ca/SODIAL-6x6x5mm-Through-Momentary-Tactile/dp/B00QLTT8ZC).
There are 2 parts for each switch/button. The mounting block which can be hot-glued down to the Gameboy body, and the switch itself which can be screwed to the block, for easy removal and tweaking/replacement.
Print with supports, and with the mount on it's side for best results. This is a delicate structure, so expect some stringing that you'll need to cut and file off. File the switch area to fine tune it's responsiveness. The tactile switch can be easily rolled into the little slot, using the extra PINs as friction anchors. I found that the open side can be easily soldered once it's in all physically in place - which is a bonus.
To prepare your Gameboy case for these switches, simply bore out the screw well from behind using a slightly larger diameter bit, to allow it some play from the inside. BUT leave the lip of the existing well, that is where the switch-base rests. You'll also need to remove some other minor obstructions from the insidewall where the mount/switch will reside.
I recommend screwing the assembly together first. ..THEN gluing the block portion, and holding it carefully in position while the glue sets - making sure the switch can move freely and click properly, and isn't binding in the screw well.
I included files only for the Right switch, so you'll need to simply mirror the files for the Left switch.