Jamie - the upper cabinets are held in place by screws through the fiberglass shell, into the top of the face frame and into the shelf support rail. I was originally a bit concerned about hanging that weight
on the roof, but once I got them in, I realized that since they are essentially one piece all the way around, they actually act like a header, and help support the roof.
Basic physics tells me that for an egg shape to sag in one place requires it it bow in another. The rigid cabinets help lock the egg shape in place, transferring the stress to the sidewalls, where it is a shear load. The cabinets are rock solid.