I used a similar design of those sliding storage draws during my Leopard Gecko breeding days. The Iris ones may be different, but, the ones I used had at least a 1/2" open gap between the top of the frame and the bin on the sides-which allows the bin to slide in and out easily. A little modification to the bins would keep Leopard Geckos contained, but, I wouldn't trust any of my 'Homer Simpson' style DIY modifications to hold out against snakes.
Also on the topic of Iris bins: I spoke to IrisUSA a few months ago about making reptile 'friendly' designs on their newer storage tubs/containers/racks or even--potentially their own specialized rack system. The customer service rep. thought I was joking at first, but, I made my case and we spent about 15-20 minutes talking on the phone. They said they had recorded my call and would forward my information to their Sales Team. I offered to field test any new designs for feedback, but, I never heard back...