I am now also one of those strange people who likes the 18x24cm format. Apart from the discussion about whether it makes sense to have only one film manufacturer available (Foma 100), the reason is that my main lens has a very tight exposure circle. With 18x24 I have the little bit of movement I can do. In addition, my Jobi 2830 only fits 4 18x24 sheets (with a self-printed spiral holder). It is too short for 8x10". I had already found two 18x24 film holders and added a few 8x10" ones. But since I end up shooting more 18x24 photos, I wanted to adapt these 8x10" holders. Adhesive strips and other options didn't seem like a good option to me. Neither did fixed conversions and gluing. So I went into SketchUp (apparently an unprofessional horror for any CAD professional) and drew a system. On one piece 3d printing was not possible as there would be overhangs and the bridges made by the slicer would be a horror to remove. So I have a main frame and an upper simple frame for the guides. In the end you just have to glue them together.
I tested these on Fidelity Deluxe and they fit well. You can just put them in and take them out like an 8x10" film. Loading the 18x24cm is analog. You have a slightly different centering, but only slightly off.
I have the STL files here and also the *.skp files for SketchUp if anyone wants to change them.
The things are larger than 20cm and fit better in/on larger 3D printers (30x30cm).