Neoprene
I made my replacement out of neoprene. Head to a diving shop and buy a hunk of nylon backed neoprene in a color of your choice (blue and black are common) and some neoprene glue, which is a contact cement. Use two coats of the glue per the instructions. You can glue neoprene edge to edge and it will be both waterproof and strong. Its really pretty easy, and you this technique allows you to do it in the field with no more tools than a pair of scissors, glue and brush, screwdriver for the hose clamp and a tube of caulk. And newspaper to protect your deck. Takes about an hour. Joseph is correct about a truncated cone, it needs to be large enough go around the mast at the top, and around the mast collar flange at the bottom. The shape before gluing is a trapezoid with slightly curved top and bottom edges. Just cut and fit, cut and fit, and when you have it, coat with glue twice and pinch the ends together. The boot does not have to be a stretch fit, make it big enough so it goes around the mast with no tension and it will be easy to pinch the seam together. To make it look neater, have to top clamp put in place with the boot going up and then turn the boot down over the clamp. Then caulk with some clear silicone in the crack between the neoprene and the mast, filling any groves in the extrusion. If there is a little extra height in the boot, that's fine, it will roll up over the caulk and hide that too. I have a canvas cover with Velcro closure and ties top and bottom over my mast boot - it hides to neoprene (not that I think that matters) and protects the neoprene from UV.Variations include a patch of neoprene an inch wide over the seam for extra strength. It sticks better on the rubber side, so attach that with the boot inside out so it is inside when you are done, and the nylon side of the neoprene is out. You can also substitute wrapping 1/4" shock cord instead of using a hose clamp. Rather than tie shock cord, lie the two ends next to each other, hold them with a pair of pliers and stretch then and have a helper whip around them with seizing line. I prefer the hose clamp; easier to remove and replace.When I started kayaking, we made our own spay skirts using these techniques, and if they can stand whitewater rivers, they can seal a mast boot.