That's a possible theory. I wasnt aware of how strong the side rails are.My guess is that the low weight rating is due to the flimsy(ish) cross bars and not actually the side bars mounted to the roof. In other words figure out how to get the weight on the side rails, not the cross rails.
I would but they are hard to find for sale and I'm unable to find a shop that can fabricate one for me in my area.I would buy or build a rack for the bed to put it on. That would be my preference.