Well, I sorted out this transom a couple of weekends ago. Being me, I went for overkill and ended up using Nida-Core pourable transom compound. Grandma sent me a few boat bucks
The result is a power-boat style transom, 2.5" thick, that will never rot. Total weight added was about 70lbs.
This was a big project. It entailed about 20 hours of work down in the lockers under the cockpit, grinding off all the bilgecoat and primer, laminating a skin the same size as the existing transom, then glassing it into the boat, 2" away from the inside of the transom. This created a void for the Nida-Core PTC to be poured into.
Once everything was glassed, cured and any wax/dirt/etc removed, the Nida-Core PTC was simply poured into the void from above decks, using a tube and a big funnel.
Here's one of my favorite DIY boat pictures: 10 gallons of Nida-Core PTC poured into a milk jug funnel taped to a bit of PVC tube. My daughter was probably thinking "Dad, you're crazy!"
I know, I know .... ridiculous overkill and expense, but now I have peace of mind and will never have to deal with rotted core or wet backing blocks!!