Swiss license plates are 50 cm x 11 cm, while those in the EU are 52 cm x 11 cm. You can simply put the Swiss license plate in the plate holder of the bike rack, but then it slides back and forth.

The holder on my ubeler bike rack is not straight, so the adapter is slightly beveled and has a gap between the holder and the plate of about 1 mm (top) to 3 mm (bottom). With the FreeCAD file, you can change that if you have another EU holder.

The adapter I have stuck with thin double-sided 3M tape on the license plate.