Scott;
I had a similar problem with a kit awhile ago. M35 Cargo kit.... never again.
It had the same problem you're now facing a warped frame. I found an article in one of the back issues of FSM and it suggested a solid surface slightly large than the frame, strong elastic bands or clamps preferably. Assembly takes place underwater after some soak time. Leave in for 30 mins then remove from water and leave clamped over night.
The other article I found adds to the above after removing from the water while still clamped use a hair dryer to re warm the area and leave clamped over night.
For the cargo box the fix may be in clamping a solid piece of wood that just fits inside the walls then clamp this to a board underneath and then immerse in warm water and follow all of the above.
Failing that... pull the plug on the life support!!!
Or... there is nothing that a couple bottle rockets can't fix permanently!
Cheers;
Gregory