I also have this problem. I've been looking on the forums trying to find a solution as well. I ordered mine pre-built to my measurements. I eventually tried the shin/calf piece on and it's noticeably larger than my actual calf. My problem is that they are too loose, and I don't have velcro on them yet, so that adds to the problem.
Personally I would prefer not to attach anything to the boots, so I might try the foam idea. But if anyone has more ideas, I'd be more then grateful to listen.