It seems to me someone on this group has actually filled their mast with
styrofoam peanuts. Not me though. If anyone else rembers any details of
this post away. I can't rember if they had internal halyards.

Don't try Parallax's spray foam experiment. You will never be able to keep
the halyards, etc. running free and if you can move them at all afterward
they will be horribly stiff (almost rigid) and very rough. Might work if
your only internal lines are wire and you use a strong solvent to clean the
wire after the foam cures/sets up. Me, I'd try a different solution.