You will only need the storm windows if caught in a major storm. If you cruise
the typical roots at the typical times, your chances of getting bad weather
are virtually zero. Not to say that it couldn't happen, just very unlikely. There are exceptions like the sail from Polynesia to NZ or Oz but for the most part, you just aren't going to need them.
I wouldn't waste the money
on anything that was expensive. Cutting the necessary pieces of 1/2-3/4 inch plywood
to the proper shape and carrying self tapping screws and a battery
powered screw gun is my solution. If I'd ever need the protection, I'll just whip out the plywood
and screws and fasten then in place. Will worry about cleaning
up the small holes afterwards.
FWIW, most boats do not have core material in the cabin sides. Think you'll find that you won't need to fill the pukas with epoxy.