Putting a lot of faith in some simple zip ties unless I am seeing it wrong. I strongly suggest a 2x2 of wood or something to support those cards in the middle. Zip ties, especially when subjected to heat non stop do not last at all, they will snap. 
Scrap'
Scrap'
Yeah, good catch. I fixed that last night actually, I'm using a 3/8" round stainless steel bar, instead, like on the rig on the bottom rack. I initially installed it without one because the hardware store didn't have any when I first built it.