An easily made and handy shop tool is a heavy bean bag. Mine is a leather bag about 8 x 8" and about 2/3 filled with #9 bird shot. It weighs about 5.8 lb (2.6 kg). I had made a previous one of heavy fabric filled with sand but, at this size, sand just isn't dense enough to make the bag heavy enough for my purposes.
Originally, I made it as an aid in clamping objects that couldn't be easily clamped with conventional clamps. It does that job just fine but it has other uses in the shop. As an example, it's often easier to bring work to a Dremel-like tool than to bring the tool to the work. I've never built a clamp for my Milwaukee Dremel-on-steroids* so for quick jobs I immobilize it with the shop bean bag. [If you do this, be careful about blocking the air vents on the tool.]
I've often toyed with the idea of making both smaller and larger versions of this but haven't yet found enough round toits. If you make your own version be sure to use heavy duty cloth or leather. You certainly don't want to leak a few pounds of bird shot onto the shop floor.
* a bit more about the Milwaukee-Dremel here...