Bahtinov focus mask to fit Canon EF-S 15-85mm
Adapter to fit Canon EF 100mm f2.8 IS L Macro
Without the adapter it will fit on lenses with 77-79mm outer diameter.
With adapter it fits lenses smaller than 77mm outer diameter.
(if more than a couple mm another adapter might be needed to keep the mask centered. Let me know and I'll see what i can whip up;) )