What's the strongest "coring" material for hatches

[FLexpat]

Dirtwheels and Daytrip are both right on.
The original hatch cores on my 23 were 3/8" thick, ~4x4 plywood squares. Unfortunately the lower skin was just tied to the vertical edges of the hatch; pretty much no adhesion area, so when it was loaded it just ripped the lower glass off the thin little edge... All the original hatches sagged like wet blankets.

For the new ones I used 3/4" Corecell and tapered the edges of the core so that the lower skin ties to the upper skin for about 1.5" inside of the vertical part of the hatch edge. Did vac infusion with overkill layup and still lighter than original. Super stiff/strong. (If I find a good pic I will edit this post since my description probably sucks to read and follow)