While trying to open an original cartridge with a heat gun (not a good idea...), I melted the original enclosure. See one of the pictures with the original one on the left.
So Fusion 360 time! This design is as close as I can get it to the original design. So it probably wil only hold the original PCB with a 'blobbed' ROM. There's not enough space for a normal EPROM.

Fortunately my original sleeve was still intact. But while I was at it, I modelled that one as well.

The front and rear part of the enclosure are printed in PLA/PHA. Printed upside down with minimal support. Support is only needed for the rear part.

The sleeve was printed in Ninjatek Fire Red. Although it fits like a glove, it is a bit stiff. It tends to push the cartridge out of the ROM slot, even printed with a single perimeter. If anyone's got a suggestion to improve this, let me know in the comments.