There are various sources for different types of mast boots, or you can cut up a car/truck inner tube, wrap it around the mast and affix it with large hose clamps and seal it with Life Caulk or a similar product.
Some of the commercial products are prettier, and, of course, costlier.
Boat: Jon Sayer 1-off 46 ft fract rig sloop strip plank in W Red Cedar
Re: Mast Boot
Another useful material for boots is wet suit neoprene with the cloth layer on the outside. This stuff is quite stretchy, not expensive and will last quite a while. You can use wetsuit repair glue to edge glue the join for a very neat job.
Jim and Ann
s/v Insatiable back in MBTBC marina, waiting for next eye jobs to be done