I did it!
Just returned from a 2-week, 2300 mile trip to Florida where temps were in the 80s and my Revo-13 sat, strapped right-side-up onto longitudinal rails, on the roof for a full week with no apparent distortion of the hull.
I used to use foam chocks attached to the crossbars which were fine for short trips but I struggled to load the boat from the side and position it on the chocks. I wanted something more secure for a longer trip. I'm an older guy and just couldn't see myself trying to load/stow the boat upside-down so I made a set of rails:
I slipped 3-inch pool noodles over six-foot lengths of 3/4-inch metal electrical conduit, made some pipe clamps from perforated steel strapping, fit them into cutouts in the pool noodles, covered the noodles with heavy denim sleeves, attached them to Mighty Mount crossbar clamps (previously used with Yakima saddles) and mounted the rails to fit beneath the grooves in the hull.
Then it was a piece of cake to lift one end of the boat up onto my PVC roller-loader (discussed earlier in another thread) and roll it onto the rails, strap it down to the crossbars (one under the Mirage drive well and the other between the rear and seat scuppers,) attach tie-downs fore and aft and hit the road.
The boat was perfectly stable and secure at highway speeds, loading/unloading was a snap and the hull was unaffected by being strapped to the rails in warm temps for a week. Total cost, other than the Mighty Mounts and straps (which I already had): about $15.00!