Branwen's mast partners consists of a block laminated from many layers of plywood, with a conical hole, made as smooth as I know how, coated with 2 pot varnish and greased with Vaseline. (I was concerned about how easy it would be to extract the mast when t he time came.) With the mast held central with temporary wedges, I poured polyurethane casting rubber around the mast, forming a watertight and slightly resilient seal.
All went well until, a few days before arrival in Antigua on passage from the Cape Verdes, this rubber collar spewed out, leaving the mast rattling in the hole. I rammed a length of rope into the gap to keep things quiet, but water flooded down into the boat during every squall of wind and rain.
I still think the system I used is a good one, but I should have paid more attention to the advice contained in PJR, that the slope of the socket holding wooden wedges should be about 1:50. My cone had much more slope than this, allowing the motion of the mast during a trade wind passage to squeeze the rubber out.
I have now re-inserted the rubber collar and fitted a split plywood collar over the top to hold it down.