First, I've noticed that my Maiden's lower deck is ever so slightly warped into a U-shape from front to back. This is causing the tops of the hull sides to buckle slightly. If I place the Maiden on a flat surface and push the front and back down simultaneously it straightens out nicely, but returns to it's former state when I release the pressure.

Has anyone else encountered this?

Any suggestions for a remedy? I haven't added 12 internal supports yet, so perhaps those plus the presence of the top deck will straighten out the sides.

Any suggestions to avoid future cardstock+foamcore warping?

FYI, I'm used UHU Liguid glue (bigger tube than their office glue, but less sticky) and an x-acto presentaion board (similar to foamcore, but sturdier)

Spray the bottom lightly with water or set on a moist papertowel and add weights to both ends and leave overnight. Be careful about not wetting it to much because the moisture in the glue causes it to warp so you want to cancel that out by wetting it just enough and reforming it. Foamcore is infamus for warping so I never base a diorama on it because paints and such will warp it pretty badly.