SSI Universal Mast Boot
I thought that I had a photo, but I don't. As others said, the top clamp goes under the boot and a bottom clamp goes around the base outside the boot. It's a little awkward fitting it, but the boot stretches nicely and can be convinced to conform. The joint on mine opened a little, so I sealed it with white silicone at the joint and around the top. Use a small bead and PUSH the bead instead of pulling. Go slow and it will come out neat. The whole job is maybe 30 minutes.
Sabre 38 "Victoria"