When I discovered two years ago, that our mast collar leaked water, I removed it, and bougth a car tire tube, which I cut up, swept around the mast, and glued together using what we call "contact glue" or "elmulsion" it still sits nicely even though it is beginning to fall apart just a little.
Ship Shops will sell you al kinds of solutions from closed mast collars you have to slip on when the mast is off og the the boat, to wrap around solutions like mine, only more expensive.
So talk to your local sjip shop, i am sure they have at leat one solution.
Jesper Milling