My dad had a pair of rubber boots that had a cut in the back which was spanned by a more flexible waterproof material. I don't remember the brand, and I'm not sure if I'm giving a very good visual description, but basically that slit would allow the boot to become narrower or wider depending on how you needed it. Once you had it where you wanted, there was a small strap at the top that you adjusted so that it wouldn't get looser or tighter. Sorry, that's the best that I've got. I'll be home this weekend. I'll try and remember to take a picture.