Seems like all you need is a rib that follows the contour of the inside of the shell that runs from bed rail to bed rail under each rack cross member. I'm seeing 3/4" tubing, a bender and welded plates on each end. Very much like a internal roll cage that fits snug under the shell. Tie the internal ribs to the topper ceiling and to the Yakima or Thule feet on the roof, with, I dunno, u-bolts or maybe tabs on the ribs. The fiberglass in a topper is pretty strong and the goal is just to minimize flex to keep it from cracking.
If it was me I would just mold in metal or wood reinforcement into the inside and ceiling of the fiberglass, but I could care less about appearance. You can see part of the 'Nest frame in this photo. There are 4 rectangular tubes, one at each corner, connected along the side with horizontal members over the windows. The whole thing is then covered in structural fiberglass.