Paintguard. I use it. It works.
I tried something that must have been very much like Paintguard even if it didn't come from Sniderproducts.com.
With my magnetic bag, it kept coming loose from the tank and bunching up under the strap that holds the magnet on my Nelson Rigg CL-135. I've got something I bought from David Silver Spares when I bought my replacment Big Lids from them.