To be placed on a Freeplay CM3. You also need buttons; I used Adafruit's 8mm soft tac switches, found here.
Drill 1-2mm holes just inside the screw posts for the leads to go through. Solder a pair of leads on one side to about 4" of light-gauge wire, fish through your holes, and super- or hot-glue the buttons to the GBA case's shoulders, then join to the board however you like (I did direct solder, but if you've got headers that'll fit, use those).
I made this in TinkerCAD, and you can remix it here.
Printed in TPU (thermoplastic polyurethane). TPU is flexible, which is a requirement of this design.
Rip out the supports
Tweezers or round-nose pliers work best for this. Grab a pair of supports, and twist, don't pull.
It's based on this thing, a GBA cart for DIY games.
For the cart cover portion, the GBA cart model was used for measurements in the construction of my own model (e.g., by placing the Z-axis through the workplane in TinkerCAD, exporting to SVG, and playing with it in Inkscape), though I did slice off and re-attach the thumb-grip.
For the actuator wings, I took several photos of the back and top of my GBA shell, and matched up the sizes to mm in Inkscape using measurements of the distance between the visible screw posts. Then, I built shapes matching what I wanted to see in those places, and booleaned them together in TinkerCAD.