If you're going to substitute, definitely go with as close in thickness as possible. Also, put the thick stuff in the middle of the sandwich, not on the outside. It'll bend better that way. Finally, use the building on plastic method that doesn't immediately attach the coaming to the hull so that if it doesn't work  you can easily try again without having to remove the failed attempt.

In fact, if you first try dry-stacking the parts on the boat using clamps, you'll see if it's going to wok at all before you're commited to gluing.

Another option is to use thin solid wood, instead of plywood, in the inner layers. Something like the pine moldings that you see in DYI stores would work. The center cores in the coaming mostly resist compression, so they don't really need the crossed grain that a plywood core gives.

Good luck,

Laszlo