I did two custom builds that I had to shelve because I couldn't fit the case together in the end, and end up breaking connections loose and generally messing them up trying to get the case together, so I wanted to do one I thought I had a reasonable chance having enough space in. So, all-in-one it is, but I still had to "challenge" myself so decided to make some rear buttons to go with the soft-tactile buttons mounted on the screen bracket. I gotta say, they feel incredible, so I'll share my stl file if anyone else wants to go this route. I originally was going to design a button well instead for the original dmg style, but went this way with a solid button and using the pegs in the case as a guide.
the buttons sit straight, and don't protrude past the case, so you can set the gameboy down without hitting the buttons. The basically feel like dmg buttons, I think. I gave mine a light sanding and coated in clear coat, and they feel super nice. I just used the original screw holes as guides, and a step-up hole drill to get to the same diamter as the dmg buttons.
here is the build - i did have some extra leds in this first pic, in the cart, but i removed them since the one built onto the power supply looked good just shining through the case, once I had it together. these are also early version of the buttons
aftermarket sparkle case from retromodding.com
a and b buttons are 2-piece from handheldlegend.com with black inner and clear outer
x and y are just standard dmg buttons
white (pads) and black (start/select) silicone from retromodding.com
glass screen protector from handhellegend.com
all-in-one board from glitchdgaming.com which was great (assuming i get the audio going)
gearbest composite screen
generic power supply
helder's 4500mah battery
8ohm 2w speaker and some ferrite beads
power switch, barrel jack, audio pot and usb port
soft and regular tactile buttons (safe shutdown on top is a "clicky" button)
i printed all the parts with some sparkly plastic i got for it.
i'm not sure which stl i used for the screen, but I think it's one in this thread: viewtopic.php?f=58&t=4981
the power switch and barrel plug bracket is off of thingiverse
i used the original external connector cover, and clipped the tabs a bit to fit a usb port.