There's a material called Nidacore that is a honeycomb material with any skin combination or thickness you want(probably other brands too). It was about $90 per 4'x8'x2" sheet but haven't looked in several years. You usually can find it where there are boat yards and builders.
My brother used it to make his roof for a fly bridge on his boat. There's a blog online where a guy and his wife built there own TC out of it to go on a Toyota.
You might be able to use it with aluminum angle and not have to do any fiberglass work. It's some strong and light material so should work well for a popup roof. I belive it was 32# for a 4x8x2".
Good luck with the project.