You might try a product called Spartite. It is expensive but provides an elegant and long lasting solution. The two part mixture is poured into the mast cavity after building a dam with clay. Once the material cures (about 24 hours) it provides a perfect seal and yet still provides a certain amount of "give". The mast is sealed at the Partners. We used it on our Taswell 43 two years ago and would recommend it to anyone.
Spartite is an excellent solution which serves both as mastboot and mast collar. For difficult to fit mast collars, like our CS 36T, where the halyard exit boxes are only an inch above the partners, Spartite is the only good solution. For most boats, there are a variety of other, cheaper solutions to a mast boot that may get the job done. Two common ones are a self-sealing tape and a pre-formed boot that you cut to fit.
You need to figure out what will work for you, I tried three different solutions for our mast boot before I spent the money on Spartite, which has worked very well.
I've posted ths before but it is cheap and works great and lasts for years!
Buy THICK 3 strand nylon line that is thicker than the gap between your mast and the deck hole (mast partner). It should be long enough to wrap around the mast 3-4 times...usually around 10' is plenty.
Get a couple of tubes of marine silicone.
wrap once around the mast with the line then pound it down with a mallet and dull chisel into the space between mast and partner.
Layer in a good thicklayer of silicone on top of this.
repeat this step until you have at least three layers and have completely filled the vertical space in the partners. More wraps are better.
Put the mast boot on and you are done for the next few years!
Hint...sometimes you need a little help getting the line wedged in as you work around the mast hole. A "come-along" line around the mast and run to a winch can help pull the mast in the right direction to make getting the line in easier.
It works great and has give so it holds up over time.