are you in it for the bass to demo and be a local hero or to compete in a class with rules
i ask because i learned alot about viechle flex in my ghetto blaster suburban build - and eventually went with entire plywood roof multi layer - and 4th order c pillar build / actually using the metal structure of the viechle as a base to attach and build to.
there is no flex anymore on my build - exept for the floor infront of the c pillar and my dash board goes a little bananas still - but taking flex out of the equasion in a loud build does a few things - one of them is the frequency resonance in the viechle - as viechle flex actually can help or hurt score at certan hz i thought it was crazy at first but it has something to do with cabbin gain im no scientist with it - but if your just trying to build a loud setup as brutal as you can get it - then yes absolutely eliminate as much flex from the viechle as you can - all that energy wasted in flex will be transfered in to harder hitting more violent feeling bass inside for the users.
just a few thoughts on diffrent ways you can go or test with. check the ghetto blaster burb build log for aplication of wood to the roof and c pillar rear area box build if you need some inspiration