You can legally use just about anything to suppress a gun as long as you pay the $200 tax to the ATF first. But since putting a potato on a muzzle does not reduce noise, it is an expensive and stupid experiment. You are better off with a plastic bottle full of mesh, but it works poorly. There is no substitute for a well made silencer. They are easy to make and legal in most states; you might want to try one if you can.
Ranb