Per the request of HoraceAndTheSpider over at the Freeplaytech forums, I created an extended battery cover for the Gameboy Advance to replace the factory cover. The new cover allows for larger/thicker batteries to power the FreeplayZero/CM3.
The Battery Cover was modeled from scratch in Fushion 360, and based around roijeric's GBA Replacement Battery Cover.
The Clip design took a bit of reworking to print properly, and actually clip into the GBA case, but the final fit is quite snug.
The Fusion 360 archive can be download here. Feel free to remix the design to fit your batteries.