I use low density injection molding plastic pellets that are leftover or scrap from our molding machine at work. LDPE, TPX, ABS are all good choices. Find a buddy that works in a molding shop and you can probably get them for free.
"Therefore go and make disciples of all nations,... "
Handgun hunter since 1979 - haven't used a rifle since!
HHI member #992, NRA, SCI.