I think it could still be done in the way you've suggested, though. It has, at least, pointed me towards a method that I can see working. I get the ferrules and get a shoe repairer to stick the heel rubber I've got on to the bottom of one with the waterproof adhesive they use then I take off the existing feet and replace them with the ferrules, the one with the heel rubber stuck on it going on the leg that is, at the moment, shorter. Problem solved.