Michael, I really hadn't given this too much thought before you brought it up.
I usually just throw a towel down and done. But if you don't want it to move around or flop onto the engine, how about these simple clamps? Use them with any towel, shop rag, or whatever. Flipped back, there's nothing to stick out and snag on.
They're available in a million different sizes from, like, a quarter-inch wide to about four inches. These are about an inch and a half and seem to hold just fine.
Keep it simple, SOC.
