and others sell a boot made by SSI. WM part # 191197.
They are easier to install if the mast is out when you put it on but that isn't mandatory. Just have to make sure to seal all the edges as instructed and it will never leak there. The deck
ring might leak but the mast seal won't.
You can make your own mast boot from a truck inner tube and a couple of big hose clamps. If the mast is on the boat seal the splice in the inner tube with contact cement. Use a bit of RTV between the rubber and the mast before tightening the clamp. Same at the deck
ring and it won't leak. Then cover over that by tying on a nice leather UV cover. It will look "salty" and last a long time.