(In a similar vein, something that also requires a 3D printed cartridge connector socket, I would also be interested in a regular cartridge slot extension cable, for use with large cartridges / expanders like the Modulon, Panasonic FS-CA1, etc. In case you’re looking for another project of this kind 
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I am interested too. Thank you for your great work. 
Well, the project is now completed. Yesterday I assembled the first adapter and I am happy how it functions. I had to modify the case's model a bit so that it allows the device to be secured in the slot with the long screw (like the original module). Otherwise there's a big chance to pull the entire adapter out when trying to just pull the cartridge out of it. This also means that the flat cable needs to be split in two in order to get the screw through. But that is not an issue.
The project will be put into our repository in April. Soldering takes 20 minutes at most. The case gets printed within 10 hours. The crimping tool for IDC male connectors may need this adapter to make the internal connection cables: https://www.thingiverse.com/thing:3504332
There's one note though - in most of MSXs with side slot, the slot 3 is expanded. So if you insert a cartridge that expands its own slots into this adapter, then the cartridge will work in the non-expanded mode. For example, with Carnivore2 you will only see the boot block + SCC and other devices (RAM, IDE, FMPAC) won't work.
And here's the slot cover:
