Went to home depot today and looked at the gussett tie that vireland posted. I think its too big to fit on either side without hitting the flipper switch or the shooter rod mounting plate. You could mount it lower but then you're just moving away from the apparent trouble area based on the pictures posted so far.

Instead, I'm going to try this. I think it'll fit near the top of the cabinet on either side with little to no modification. Might need to move the grounding braid out of the way, but that beats taking a dremel to something.

http://www.homedepot.com/p/Everbilt-2-in-Zinc-Plated-Heavy-Duty-Corner-Brace-2-Pack-15442/202034088

My cabinets are fine, this is just cheap insurance. I'm sure Stern would make any real issues right like they are with playfields, but I'd gladly drop the 5 bucks extra per game to have a little peace of mind.