This OpenSCAD library is considered to be a starting point to create your own custom 19" rackmount which can be easily printed.
Use the customizer variables or modify the "panel" module in the .scad file to add a cutout in the front that fits to your device(s).
The optional stabilizer (you need two, the second one needs to be mirrored in your slicer software) can be used to enhance stability of the whole rackmount.
I am going to print some prototypes in the next days, so hopefully there will be some real-life pictures soon ;-)
Update: Did my first "production print" to integrate two Ubiquiti PoE injectors into the rack. Worked quite well, though I would suggest to move the "connection holes" further to the front (the holes that connect each of the rackmount segments with each other). When adding pressure from the bottom (which won't happen in a rack...), the segments will move apart and create a gap at the top (especially when using PETG...).
It might help to increase the wall thickness to reduce flexibility of the whole rackmount.
1st small prototype: Mounting dimensions and hole positions are okay
1st small prototype: Side view
1st small prototype: the bottom side
1st practical use: Make Ubiquiti PoE injectors become enterprise hardware