Actually, no worries. I just gutted a GBA game I hated (Sword of Mana *shudder*), chiselled a lot of the inside out and packed my M3 in there.
Funny related story: I've had my M3 Lite for about two months and every time I wanted to use its MicroSD card I would simply push it in or pull it out. While I was packing the M3 circuit board into the GBA cartridge, I heard a few clicking sounds, and when I tested the MicroSD card's movement, I found that the slot is spring-loaded.
The MicroSD slot was spring-loaded all along, and it never worked until now! I must have tripped a catch or something with all the pushing I was giving it.
Oh, free tip: if you want to remove an exposed tri-wing screw (like from the back of a GBA cartridge) but don't have a tri-wing screwdriver, just grab an appropriately sized flathead screwdriver, angle it and slot it into one of the wings on the screw and twist.
I'll walkthrough-ify this conversion in the following week.