I ordered a new original boot through CatalinaDirect.

Quite an easy job to replace the boot with the mast off. 

After removing the old damaged boot and cleaning the mast, I put the new boot in a bucket of hot water and with the "heated" boot is was easy to slide it over the mast to the final end position.

When cooled down the boot had a tight fit around the mast and no concerns and the supplied stainless hose-clamps are more for the show than that they have a real functionality.

I am aware that this needs to be done with mast off, but to our experience taking off the mast is an easy job which can be done within an hour (we do this exercise every year when taking out for the winter), so within a day you can get the best future-proof solution instead of struggling around with other so called "mickey-mouse" options.

I replaced mine with the mast up with this: https://www.fisheriessupply.com/sailing-specialities-universal-mast-boots/91200000
I'm not sure which size I used, but I just measured the circumference of the mast and collar to pick it. Following the instructions that came with it for installing with the mast up, I cut it down the side and then trimmed the top and bottom down to where you have a bit of overlap at both the mast and collar. It comes with a rubber cement type glue that you use to glue the overlapping section. Then I put silicone on the mast and collar where the boot ends up and secured it with hose clamps. I needed an extra hand to help stretch it evenly around the collar while I tightened the clamp. I did this a couple years ago and haven't noticed any leaks.

Richie,
Richie,
I had my mast stepped after the winter having my rigging replaced. The old boot that was on was an accordion looking type used for after the mast is in place, that I had installed to stop the leaks.. It came in a kit ( I'm thinking Catalina direct) with stainless clamps, glue and sealant. This spring I used the hard preformed boot and what a pain in the butt to get it to fit, and the wedges in there too. You cannot slide the boot up high enough to install the wedges so off comes the boom vang and bracket! Wew! That wrap or similar would probably do the best at this point. Checkout the link.

https://www.catalinadirect.com/shop-by-boat/catalina-42/rigging/standing...
 

For many years I've led the mast stepping crew at our self-help club in Toronto, so I've had a lot of exposure to different keel-stepped mast installations.  I completely understand that, 'intuitively' (human kind's most natural go-to methodology for solving what appear to be problems at first), it appears that the mast going down through the hole in the deck requires some 'help' to stay centered in the hole.  Our PO also had some carefully carved pieces of wood and plastic designed to be stuck down the sides of the mast.  However, for many years now I have ignored whether the mast appears to be a bit port-starboard, or fore-aft in the hole when stepping and wait to tune the rigging.   Seems that the turnbuckle that is intended to pull down the deck down around the mast and ensure that the shrouds do not squeeze the hull inwards is also super important to this process.  Once I tighten the outer shrouds (read the C36 owners' manual), the mast centers itself and no blocks, chocks, Spartite, etc. is required.  I can then slide the Cataline Direct pre-formed white plastic boot down over a bead of clear silicone and use two large hose clamps to secure it above the aluminum deck collar and around it.  Consider giving it a try and ditching the 'centering aids' that 'feel' like they are essential to the process.  The mast still leaks down the center of the mast into the bilge, but we have no leaks around the boot.