I don't ever plan on using the sail option, so could I not just shorten the mast insert and put it back in.....the floor of the mast insert is still there? Or does the mast insert need to make contact with the bottom inside of the hull for some reason?
You could. However, it wouldn't be difficult to repair it. Benefits are:
1. it also acts as a stiffener or structural support (deck to hull)
2. if you cut it short it will dangle and leave a hole that needs to be filled (is only sealed near the bottom)
3. if you remove it, you still have a second hole to fill with something
4. You may never use it, but someone one down the line may -- it would be harder to replace later than fix it now.
Good news. Rummaging around the garage I found a couple of Hobie blue sample discs (see pic below). They measure 2 3/4 " diameter. I'll be happy to send them to you if you decide to go that route. You can PM me with an address. Hobie also makes welding rod in various hull colors and I don't believe they charge for it (maybe shipping?). Some is shown in the picture that is some "dune" colored rod I use occasionally. If you decide to weld, practice first -- if both sides aren't heated before you use the rod, it might lack bonding strength -- could pop off.
Finally, I have some Goop and am running a sample bond with it on Hobie's PE (also in pic). I, a firm believer in trying something first-hand before assassinating it. To that end, I'm a little disappointed in some here that would make blanket statements about products without at least some investigation. The epoxies I have recommended to you I have used on several occasions in high stress areas on Hobies. If you care to open the first link I posted earlier you would also find a test sample that I had to chisel apart. I'll report the actual results of the Goop test tomorrow after it cures. If you have doubts, I recommend you call 3M and Loctite and discuss these products with them before deciding how to proceed (as I did). I wonder if any "doubting Tomasas" have actually tried these specific products before pronouncing them ineffective.
If you decide to go with the preheated cutting board, I would think you would want to have the boat standing by close enough to the heat source to install it before the patch cools. Have a burner plate in the garage or bring the boat in the kitchen when the wife is away??