Consider using 3mm EVA foam tape if your horse needs additional heel protection. With its adhesive backing it can be cut in strips then folded over the heel strap for customized protection. It's an inexpensive alternative to gaiters and it allows targeting of specific areas that need extra protection without adding bulk elsewhere.
Sold here in strips 6 inches long by 2 inches wide. Note that it is available by the role elsewhere online (e.g. on Amazon).