Reinforce a race car roof

I have laminated a couple of layers over the existing steel roof, and I plan to cut away all but two inches around the perimeter. If I glued the composite roof to the flange first, it’s sure to fit, but then reinforcing the ceiling would be awkward. I was thinking a big X shape foam core going cross corners would be a good shape that would allow for helmet to ceiling clearance. Without the car upside down, laminating over the core seems too difficult. I’m nervous to reinforce before gluing the roof down, for fear that I twist it enough that it won’t fit. I’m open to all suggestions.

Aesthetics is not a consideration. I’ll deal with it later.

Just to follow up on what I ended up doing on the Kevlar/carbon roof, here are some photos. I forgot to mention that initially the entire steel roof was taped over with packing tape so the composite wouldn’t stick to it. After curing, I released the composite roof from the taped steel roof but left it in place. The composite was made of 10 oz Kevlar and 15 oz carbon, and was very flimsy. With a suggestion from a friend, I reinforced a cardboard box, then stuck the box to the composite roof using masking tape and hot melt glue. This way the roof could be taken off, flipped over, and reinforce the ceiling with it upside down. Half inch foam was used as a core, and three or four layers of carbon for a facing. After cutting out the steel roof, the carbon roof (and box) were glued onto the steel flange. I was somewhat amazed how well the new roof fit the steel flange, thinking that it would distort somewhat, but it didn’t.

The steel roof weighed 13 lbs, and the composite roof, after subtracting the weight of the box, weighed in at 4 lbs 5 oz.