Hi Yellow66,
My plates are the same format, GT66US in premium plate size. I had the same issue and just got a short piece of black (suited the car) conduit about 1 cm or 1.5 cm in diameter and cut it about 2 or 3 cm long and used it as a neat spacer between the plate and the panel. Looks and fits best, won't rust or cause paint problems easily etc.
Cheers Bee