Hi Mal,
Nice to see that you have adapted the balloon idea! Since some of the M60A3s from the Reforger exercises had black mantlet covers, I might just leave mine unpainted:
My biggest worry is the rubber deteriorating over time. However, it does give me enough time for look for a more suitable cloth in the meantime. In that case I can simply rip the rubber from the styrene and glue on the new material.
Nate