I've always bought stainless steel license plate screws at my local Ace Hardware. Since they have many different lengths, I can touch the screws to measure them unlike at the big box stores (Home Despot, etc.).
Hi. I also found it somewhat challenging to attach the plate without damaging the fiberglass. I purchased 1" stainless bolts with washers & lock nuts. The bolt head is behind the plate and the washer & nut is on the outside. Make sure the bolt has a hex head and not one slotted for a screwdriver. A small open ended wrench holds the bolt while nut is tightened. Once done, you shouldn't have to deal with it for years. Good luck.