Sorry SIG, but I couldn't get access to that page. But I'm glad, I guess, to hear that others have the same problem, and I don't have a case that's out of tolerance.
The problem appears to be that the front edge of the case has two little molded projections to make contact with the surface, but the point of contact at the rear is the battery retaining clip. So it's a 3-point footprint. (Even thought the battery clip appears to provide 2 points, they are close enough together as to be as good as one, compared to the front. And since the clip curves upward at the back, the radio wants to "roll" back on it after it leans past a certain point.)
The solution I'm going to explore is to find the correct thickness of sticky pad to place on the aluminum edge of the chassis that extends from the sides of the battery clip. If the thickness is just right, it will match the height of the front support nubs, and provide a 4-point contact.
Time to get out the feeler gauges and the digital micrometer! I'll share photos if I come up with something useful. Why they didn't build this solution onto the chassis itself is beyond me.