Finally got started on cutting the core material.  It is a bit slower going than I thought it would be - but I am learning as I go and getting faster. Things I have learned:

1. The multitool is about the only tool you need
2. It is a lot better to cut and fit smaller peices (1 x 1 foot sections or so) of the balsa or so when you have irregular shapes and such to deal with.   My deck is pretty butchered up this was the way I was getting a good fit.  I cut small cardboard templates as I go then I cut the balsa from that.
3.  The pro-balsa core material has rectangles instead of squares - so for a good fit you have to have the blocks running the right direction.  I had intended to work from stern to bow - i cant really do that - i have to go from port to starboard.   I would have cut out the original deck differently if I had known that ahead of time...

I have told everyone we are going sailing July 4th weekend no matter what - so weather permitting I am picking the pace up from here on!

Figuring this weird arrangement took a while - but saved me from the sun of a 90+ degree day!

As far as I got with the pre-fitting.

Done for the day - thunderstorms came in a few minutes after this.