I used some flat plastic sheet and some strips of 1.2mm sheet metal to hold the plastic sheet in place, it stood up to sand blasting and was only removed once the paint was complete.

The plastic I used is similar to the soft plastic you find on an ice cream container lid except much thicker. We receive it as covering material for some of our merchant steel, great stuff. Perhaps ask your local plastics shop if they have anything similar. Cardboard would also suffice, flat card.