I spent some more time with my Thule XT Sport. I tried cutting some plastic inside to lengthen the front clamp travel by ~.5". However, that wasn't enough. While the rear clamps have enough travel (aft-wards) the plastic 'feet' of the box interfere. A deeper, more careful re-read of the Thule documentation suggests that the max (or min) bar spread specs should NOT be interpreted as bar center-line to centerline (as per one of Thule's drawings, but rather bar extremes. In my case, the Model Y bars are 38.5" from front of the fwd bar to the the back of the aft bar. thus the 36.75" spec of the Thule XT sport won't work.
as I'm not really looking forward to reshaping (read: melting) the box to make the feet fix around the bar I may simply try to find another option.
at some point I'll re-read this thread for ideas, but I think there is at least one Yakima box that has a max bar of greater than 38.5.
I think Nate (post 53) shows a Yakima. I'm curious how the Thule Motion Xt XL with spec'd spread of 36 7/8" worked for MikeInFL
edit to add link to Thule fit reference:
Thule_Cargo_Box_Hatch_Clearance