I poked around Expedition Portal last night and found one guy who say his fiberglass cracking, another who said just a topper was causing the bed to spread. I've been working on an idea for an external frame that hopefully won't look like a contractor, but the dilemma is how to get it sturdy enough to not flex or move, yet keep it sleek enough to not just be a contractor ladder rack. The other downside to those is how far out they stick, when I had a 2wd Taco I had one and was always catching my hip on it.
My other thought is just scrap that idea, and instead do an internal support cage of some type-so you've got a couple bars below the rack rails, and some legs that run down to the wheel wells/truck bed? In that situation your topper isn't really doing anything other than acting like a fiberglass washer for your RTT support system, and keeping the load from swaying side to side.... but all the load is transferred down to the truck bed. I kind of like this idea better.