Good morning Stuart:
You could purchase some cheap personal flotation devices (preferably red as its a magical bullet proof colour according to certain drug addled thugs in West-Africa) and strap or glue them to your prototype!
The FN C1 generation weren't so lucky as to have rubber rifles... we got Ex-1 and 2 rifles (
http://www.falfiles.com/forums/showthread.php?p=3272405) as DP (Drill Purpose) for PT and watermanship training. The jump school also used them for Basic Para. Still had to keep the damn things spotless and woe betide the idiot who dropped his rifle!!
Cheers! Mike
