Mnay ppl have complained about slapping of halyards and wires inside
their masts and are looking for solutions. Strangely, this has never
bothered me but I may have a solution.

Get several cans of that expanding foam (product name "Great Stuff").
Every 10' or so drill a tiny hole in the mast large enough for the
nozzle to fit. Push the nozzle in far enough to squirt a dab on the
opposite internal mast wall, on the side wall and opposite wall and
withdraw the nozzle to put a little just inside the hole. Allow it to
set. Push the nozzle through the dried foam at the hole till it is
above the four foam spots and inject enough to form a blob filling the
cross section at the mast. It will be held in place by the four
hardened blobs. Let it begin to set but work th halyards up and down
as it sets.

I have not tried this but might if the slap ever bothers me.

DBO