Here's the rubbery mat, and some gravel. The piece of mat gets cut down to fit. I consider using a rubbery contact adhesive to glue the mat to the sock (and through it to the board). It would have the advantage of being more flexible when dry.but the disadvantage of decaying after a year or two, so I stick with the wood glue. (Glorious pun!) I try to let it soak through the sock but not get too much on the under side of the rubber mat - the more flexible if remains, the more it will grip.