I was playing around with a rear door off an old postal Jeep today:
And I was also thinking more about the split hard cover I plan to make in fiberglass:
And then the two ideas came together... the same half-covers I would make in the photo above could also be used as side panels for a hardtop, just bolt them in place vertically...
The postal Jeep door is slightly tall for the hard cover sides, they'd be 24" high (because they're split to cover half of the 4' wide tub when lying flat), and the postal door is also basically 24" high above the tub (I cheated the side panels a little taller in this drawing to make the door almost fit), so a different door would have to be made up in fiberglass, also a flat roof panel would have to be made up to cover the center above the two vertical panels, but those parts would be very straightforward to make.
The sides are more accurately scaled in this photo/drawing:
The tub being only 6' long, I think this hardtop is a little short to call a camper top, but perhaps if there was a canvas extension to cover the drop-down tailgate, then you'd end up with almost 90" of horizontal space...