This 3D-printed board is identical in size and shape to the original sensor board in the NES Zapper gun. Just glue your Arduino board down and you're good to go!
This 3D-Printed part makes it much easier to mount your power switch (as shown in part 2 of wermy's all-custom-and-aftermarket-part build guide: http://sudomod.com/game-boy-zero-custom-parts-guide-part-2 )
It can be used
With a 3D-printed screw bracket you can avoid the whole process of fiddling with the original screw posts and epoxy and all that measuring to get it just right:
These help hold your buttons in place, prevent them from rotating, and limit how far they can protrude (so they match how far the A/B buttons can stick out)