Okay...I can't imagine filling the holes to add more than 1/32 of an ounce. What about scott-lite? It'll make your design fun and safe. Comes in several colors. try using crepe tape. It's really fun, thin, bendable tape. It's used to create "sketches" in auto design. Also pinstiping tape, which can be bought at autoparts stores would work, which also comes in relflective varieties I hear.

Or just get vinyl decals cut.