Hello 
The provided .stp files are incomplete for the bottom chassis, so mounting points and internal components are missing. My own project was designing a 3D printable lower chassis, which is being tested at the moment, and I hope to revise the design further. In this design, I used the official motherboard case to help align the internal components with this frame, so from a dimensional perspective, it should accurate.
I don’t think the official bottom shell would print well… It has very thin walls, at 0.8mm for long sections, which has proven to be a point of failure in my design too, even though it is only for a short distance. There does seem to be a lot of interest in designing a 3D printed enclosure for the Framework 13 though. It would be good if these efforts could be standardised, and enable a wide choice in custom accessories.
Zdeněk