Whoopee cushions. I know Silly Putty sounds bad, but really, PFS. Playing the 'cello is a serious pursuit.
Here are photos of my recent acquisitions. Much easier to explain this way.
The "sitting cushion" looks like a Frisbee from the top, but you can see the little feet around the bottom edge. Unless you're into some kind of spikey buttocks massage, I would recommend sitting on the smooth side.
It works really well for filling in the indentation and backwards slope of the cafeteria or office-style chairs many of us have to put up with in rehearsal spaces.
Here I have divided the "egg" of s.p. into approximate thirds. I used one small third to make a little saddle over the top of the bow in the middle of the frog.
This is how the s.p. looks after it is flattened into an oval, about 2 mm thick, before being placed on the stick.
A quick bow hold to show you where the s.p. falls under my hand, approximately. It's just there to add friction, so it doesn't need to be a specific thickness (just not too) or very accurately placed.
This is what happens when you try to perform any activities involving Silly Putty in the vicinity of a kitten.