Fabric used as sheathing.

[GregT]

Something I've always wanted to try is using layered HDPE as a shell. Collect old shopping bags (the thin noisy ones you get at the grocery store) and layer them to create a protective barrier. This would be a very cheap option, as most people have lots of these bags laying around, would be lighter, and may stop must blunt objects like balls or goals (anything sharp will cut through it).

I really have no idea how this would work, but it's probably worth at least a test.

Greg