All sign shops carry mask material. It's a low tack vinyl that can be easily removed. If need be, it can be applied with the surface wet. This is for correct placement.
Walk in any sign shop and ask for mask scraps. If they have them you'll get them. If they don't, they will save some for you when they do their next mask job.
Might have to wait a bit, but they will be for free.
Yes, I have a sign shop and make model graphics. Build models too!
Charles