Hummmm....I envisioned the four pieces a bit differently. I kinda like the idea as pictured. However one more idea, get out the router make all the joints interlock versus just butt up square. Again I'm assuming you are still looking at using 1 1/2" thick material.

Yes, ideally I'd like to use a single sheet of 1.5" Penske. I agree that interlocking similar to a puzzle piece could be effective in strengthening the lay up. It's either Coosa or Penske that has some examples using a puzzle type cut to interlock. I just had a chance to check out another old Seacraft (72 20' Safari) in my yard that has had the outside transom skin removed and it gave me a little concern. I noticed the inside skin is not really perfectly smooth and could kick the individual pieces off flat when I go to clamp the whole mess up. The outside skin is .25" and strong enough to hold the old motor on, so I may be imagining something that won't happen once that skin is clamped onto the top of the new core. I would suck to be knee deep in wet epoxy attempting to fit in a $600 core that won't lay flat against the inside. I understand the 18oz mat and thickened epoxy will help fill in the undulations and potential cavities.