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.