FWIW I stopped using my horse’s BoT mesh sheet because it was making her withers sore when used just for 45 min or so in the stall before riding, and rubbing to the point of creating nasty sores when left on overnight. I’ve seen the same in several other horses.
It’s a poor design and hasn’t been updated, to my knowledge, in years in spite of plenty of evidence of these problems. I suspect they wouldn’t cause such problems if they stabilized the spine with a line of webbing (as many fleece coolers have).
My horse’s is in her normal size and runs a bit small, but still pulls back and makes her sore, so I wouldn’t expect sizing down to be a solution for the rubbing.
Several folks I know have been happy with Schneider’s Lux as an alternative to BoT that doesn’t create sores, but I haven’t tried it (or the Hansbo option you mention) personally.