About 5 years ago I provided Van's an analysis of the canopy fit problem that was directly related to the width of the cabin. This is a manifestation of the same problem.
It appears that there is no structure to control the upper cabin width at the spar station. This leads to the canopy track fish-mouth problem, plus a need to (often) cut and weld the canopy frame to make it fit.
I did a survey of several RV's with sliders and measured the cabin width at the roll bar and interviewed the builders. There was a positive correlation between width and canopy fit problems. Total width variation was in the order of 3/8 to 1/2 inch as I recall.
I wish Van's would publish this dimension so builders can fabricate a jig when building the fuse. Better yet, they should use a jig at the Quickbuild factory.
See
http://www.vx-aviation.com/rv-9a/Tips.html