When I shot RF I ran a piece of electrical tape over the top of the finder, down the side, and across the top plate for an inch or so. Folding over the last 1/4" makes a pull tab to make it easy to change finders. That worked fine for Leica finders with metal feet, and later, with plastic C/V finders I discovered that fleshing out the bottom of the foot with a strip of masking tape added sufficient traction.