You can buy sheets of rubbery material with sticky backing and metal powder embedded in the rubber. One supplier is WarMag - people use them as a surface for pu

You can buy sheets of rubbery material with sticky backing and metal powder embedded in the rubber. One supplier is WarMag - people use them as a surface for putting magnetic-based figures on. I came into possession of several sheafs of the A4-sized ones, which now serve as "generic surprisingly heavy objects".