Normally the Gameboy's internal boot ROM will check the logo and reject any game where the logo isn't correct. However, this memory area is read twice, once to copy the logo to video RAM and once to check that it's correct. So what a game can do is to activate one memory area, then another memory area with precise timings to both let the Gameboy boot ROM copy the custom logo to VRAM and pass the check.
Other cartridges that do this include Mega Memory, a 3rd party PDA type application and Nanoloop One, the funky PCB cartridge.
As always, I'm interested in getting thing like these as donations for research/preservation. Especially since you just want the plastic, and I'm mainly interested in the PCB. Though it would sort of feel like a shame to divide a rarity up into parts. Though, first you could it the cart PCB you want to put in it even fits well in the shell. Sometimes the pirates are a bit off on their measurements. But it's your cartridge and I'm asking to have something for free, haha.