Just remove the left rib all together! (aft of the sub panel, of course) My inspiration came from Scott Solber'g Super -7 who has 2 GRT screens. He removed the rib completely as can be seen in this picture. He reports no problems after flying with an IO-540!
Here is my aircraft (will have 2 GRT screens as well). I was experimenting with stiffening the top of the panel with some angle. Not sure if it needs it but it is an idea that appears to work. Here is a picture from my website with the angle loosely in place:
