[FTC]: Concrete?

[DonRotolo]

I see your point Andrew.

Of course, 60 lbs of concrete costs about $2.85, and that weight is without the water. You can cast it in a cardboard mold backed by sand to keep it from moving. Wrap it in duct tape (or cast it in a milk jug or similar) to avoid any bits flaking off. You can even cast in a way to bolt it to the robot frame.