I used the Martin/Classic Equine neoprene shims. I used it on my gelding that had some atrophy. I used them for a few months and did a lot of long trotting. He filled out really good. You don't have to use any particular shim though - I realized after I bought mine I could have folded up a kitchen towel or used sheepskin.
I went to wal mart and got thick foam. The great thing is when they lose their ability to bounce back I can throw them away and cut another piece. And it is cheap!