There is quite a bit here on the forum if you search "H37C mast boot".
My solution was to have a band welded on the outer diameter of the the aluminum plate that bolts to the deck and functions as the mast partners. This permitted me to fabricate a waterproof boot (I used vinyl but canvas would work too) that could be hose-clamped top and bottom. I used the pattern suggested in "The Marlinspike Sailor" by Hervey Garrett-Smith. There are more 'modern' systems available as well - one consists of basically really wide, white, self-amalgamating tape. My friend with a C&C 37 swears by it.