Color Computer Mega Mini MPI Extender - This will prevent the inserted cartridges from wiggling around too much.

The photos should give you a pretty good idea how to put it together. The screw mounting is for M3x10mm button head cap screws and comes in two flavors (m3 heatset insert, or m3 captured nut).

IMPORTANT: Be very precise and careful when measuring/marking the screw holes. I suggest lightly clamping the printed model in place and then using a thin sharpie to mark the hole placement. I may printable template later on to more easily mark the holes.

Note: Since the photos were taken of my first print assembly, I have adjusted the four screw hole mounts to be inset by another 2mm to ensure the screw heads don't interfere with the plastic ridges on the Mega Mini MPI case. My first print worked, but the screw heads were very close. The revised models should work fine and leave plenty of clearance for screw heads.

You can use one of the pre-assembled models, or you can also "stack" and group/merge the parts located in the "Parts" folder to create your own customized version. This can easily be done in a CAD program of your choice. TinkerCAD should work fine.

BOM:

4x M3x10mm BHCS screws
4x M3x hex nuts (only for M3 captured nut base)
4x M3 heatset inserts (only for M3 heatset base)

Print Settings:

Filament: Polymaker ASA
Hotend: 260C
Bed: 110C
Chamber: 40C
Infill: 20%

Based on original model:
https://www.thingiverse.com/thing:6786612 (by https://www.thingiverse.com/Flutterball)