well i think the best way would be to fill it with something heavy. then connect it to a thick steel plate. like screwdriver suggest, just like that vice you have on a steel plate steve..
u could always bolt 2 discs together if u need extra weight,
Aha! My favourite workshop hack: the portable vice.
Who is this Steve person you refer to? I am Screwdriver! Anonymous agent of the internet.
Screwd.