Parts are here! Got the Pi zero loaded up and tested for basic Retropie software and a few roms to make sure it works. Man, this thing is tiny after working with the larger Pi 1 and Pi 3 models. Began modifying the front case. Initial holes were drilled for the x and y buttons using a jig that came with the controller board. Used gradually larger drill bits to widen the hole, and all went well until the last part of the second hole. The drill bit seized up on the case and whipped it out of my hand and started it spinning. Put a wider gouge on one side of the y hole than is needed. Not super bad, but I still notice it. A dremel with a round flat disk cutter blade thing was used to do basic cutouts for the larger screen while keeping about 2 mm away from the edge I wanted to keep.
After that a large flat file was used to clean up the holes.
Next I checked the fit of the controller board with the new button holes and the monitor. I'm just holding them in place for this, I haven't glued or screwed anything yet.
I cleaned it up from the shavings after this. The monitor was glued in place and the connecting cable clipped down for easier handling. The upper buttons were very loose originally as well. This would not work for any kind of gaming using the top buttons, so I switched the black ones up to the top because they were larger slightly than the ones harvested from the SNES controller. The buttons spun and wiggled entirely too much. In the casing for a SNES controller there are the same style button holders as there are in the original gameboy case. I used a dremel to cut out a double set of button holders and glued them into the gameboy case.
This holds the buttons very nicely in place, and I moved the purple buttons back up above the black ones. Now the problem was the added depth of the button holder made it so that the buttons didn't stick out far enough. Using a file and a screwdriver to stick in the button I filed down a lighter purple button, and filed off the tabs on the darker purple buttons. I glued the filed down button to the back of the darker purple button. This worked much better. You can see the approximate depth needed versus the black buttons here.
After this I wanted to give the glue some time to set firmly in place, so that's all for right now.