If you haven't already done so, look here:
http://tnttt.com/viewtopic.php?f=5&t=50969 You can build a pop-up roof to any size you want. To help keep it drip-free, build a docking lip that the lifting roof section docks over. Add weather strips and seals along the edges of the lifting roof where it touches the stationary roof and to the lifting sides, where they meet. Reinforce the opening with a light framework of 1x2's. Take a look at Cracker39's trailer. You might like that one.
viewtopic.php?f=50&t=41606