Consider drilling a series of 4" holes instead. You'll retain most of the shear strength, and get some ventilation. Diagonal braces are more effective but more work.

Consider drilling a series of 4" holes instead. You'll retain most of the shear strength, and get some ventilation. Diagonal braces are more effective but more work. T brackets at each rear junction are a possibility, but probably not as strong as the backboard with 4" holes.