I put a 2" piece of self-adhesive tapered cork bar tape under the thin insoles of my Sidi Dominators, right over the cleat area. Feels like it might get painful to walk a long distance that way, so maybe that's too thick. The Sidis are quite stiff-soled, but they do have a noticeable hollow where my SPD clone cleats attach. I've got a strenuous rain-ride scheduled for tomorrow, so I'll let you all know how that worked.