I have repaired 3 or 4 mast steps, I simply use 3/4 exterior plywood, but I made sure it was dry and several sealing coats of epoxy. I made the replacement pieces the same as the factory , but made them a little bit beefier. They were glassed into place with chopped mat and 10 oz cloth. On two of them I replaced the whole floor also, one I simply patched. On my boat I left a a rectangular cut out aft of the mast and use it as a hatch to put a small anchor line. Make sure you plan for drainage