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There are many different sizes screw-mount soft rubber feet shown in the following link. They can be found at Home Depot, Lowe’s, Walmart, Ace Hardware, and so forth:
Black EPDM rubber is sold in rolls of varying thicknesses. I've used a thicker roll to make a roof for my shed. It's commonly used under shingles to prevent damage from ice dams on roofs in cold climates. It would also provide good friction. The advantage of not being pre-cut into feet is you could also use it for other things. Roofs... pond liner... protective padding.
I have used rubbery shelf liner from the Dollar Tree + heavy duty double stick tape. I have to replace it every 8-10 years because it loses its grip. Total cost back then= $1.50. Total cost now= about $3.00. Enough to do several easels.