$0.02.....
Would putting the heavy stuff INSIDE the X, and the light stuff on top work for you at all?
I do environmental work...and 4 wheel into some hairy spots carrying a lot of heavy test equipment...I try to put the heaviest stuff on the floor behind the seats, etc....anything to help me through the off camber with out needing a sunroof by the gas pedal...
So 50 lb lead acid batteries, hi-lift, spare, tools, cans, etc....go low....First aid/survival kits, tarps, ropes, clothes...go in a waterproof bag on the roof, and or on top of the heavy crap inside...
Even if its impractical for ALL of your stuff...maybe a gas can for example, the concept goes a long way towards keeping your knees away from your ears as you hang by the seat belt....
- Lance.