I see your a new member, Welcome to the Forum!
I use the exact same rolls as tomsteve above.
I have a 18" long, 8" wide piece of melamine coated particle board I stick different grits to.
I sand all my tubes and fins on that. You just turn the tube as you do a criss cross action.
This may not be solving your cramping hands problem though.
But I can tell you from experience, a vibrating sander will make you cramp up faster.
You should see you doctor about it. There may be a more serious underlying problem causing this.
One of them oscillating tools are nice. They can cut slots in tubes for TTW fins real easy.
Also can be used for cutting off tubes, nice for fiber glass and carbon fiber tubes.
Also cutting out fins. If I had the money, I'd buy a Fein.
Just make sure you get a variable speed tool, either the oscillating or hand held sander so you have greater control.