My X Y L1 L2 buttons are on the backside to stay with the stock look in the front, and i mounted them in the battery cover, so it can easily be reversed by relacing it, without leaving permanent damage on the Game Boy case.
I choosed the SNES controller for its button layout, and to reuse the buttons and the rubber contact thingy.
After hotglueing it to the battery cover, to use it as tempelate, i drilled the corresponding holes till the buttons hat a decent fitt.
Then i mounted M3 screws with superglue after removing the little walls on the backside of the battery cover.
The screws allowed me to mount the PCB without it sliding all over the place, and mark my contact points with a pen, thrugh the holes.
Here is a simple scematic for 4 buttons with common Ground.
The soldering job:
1. Wire up PCB (hold the cables in place with duct tape)
2. Solder the wires on the board
3. Enlarge your contact surface by spreading solder over several solder points (i used 3 points for one contact)
4. File down the solder joints, till it is smooth. At the end you can barrely feel them.
!! Dont exaggerate or the solder will get to thin or break and loose contact !!
When finished, just screw the PCB to the cover with the buttons and the rubber contact thingy.