I use an REI UL cruise and don't have the problem with the back fitting so tight that I sweat. I was thinking of bending the stays for a better fit to the curve of my back but decided to leave them as they are. (didn't want to screw up a new pack) the pack rests against my back in only two places at the top and around the waist belt. I was good in 95 degree temps.
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Enjoy your next trip...