I'd have been tempted to put another Lego connection on the bottom (a bottom mounting one even) (another pair of 6's maybe?) so that the Pi cases could truly stack, or be more easily incorporated in a Lego project.
I agree that having it snap together like Legos would be ideal. Underside Lego compatibility would be great too. But molding reliable Lego studs is difficult to do. The case will stack reliably as the underside has location features. You could then secure them together with zip ties.