Summary

These are frames to hold wet painted panels for transportation.
Wet panels are placed in the frame with the painted faces facing each other. Rubberbands are stretched around the frame/panel assembly to keep the panels in place.
They require a large bed printer (mine is 300x300mm.
Hopefully it is clear from the models which face should be placed on the print bed. When placed correctly, the only bridged sections are the relatively short ones in the corners. On most printers, those bridges should be easily printable without supports.

PETG is recommended since they'll often be outside or in a closed car.