Have you tried a heel cup? They are plastic cups that fit around your heel only in between your sock and the shoe. People use them for extra support or cushioning , but they might work for you by filling up the extra space in the heel of your shoe. You should be able to find them as sporting goods stores or athletic shoe stores.
Good luck. I too have wide feet (but luckily I have a normal heel) and I know how hard it is to simply find athletic shoes in wide widths.