Yeah, that would be hard to completely avoid with that kind of shape and PETG. (Travel threads between the walls.)
I wonder if using enough outlines to fill your side walls instead of using infill wouldn't cut down on it.
Try doing that and changing the
Thin Wall Behavior on the
Advanced Tab to Allow Gap Fill when Necessary (selected) with
Allowed Perimeter Overlap of around
40% or so.
(Look closely at the Preview when you slice it though) - that technique can result in double walls if you don't have enough outlines to completely fill up the gap between the outlines.
Pretty normal for PETG, unfortunately.
