We recently finished our Kallax hack. We used polycarbonate for the back because its not as brittle and we drilled holes in it with the circular saw to put vents in, we then used Perspex for the sliding doors because its cheaper and lighter than glass and it also doesn't rattled in the runners. I will ask my boyfriend which company he used as we ordered custom sizes if you are interested? I'm afraid I cant help out with glass though if that's what you want to use.
The short version for ours was: Get the metal bar we used to brace the middle because without dividers it loses structural strength and sags in the middle. Get polycarbonate sheet for the back, cut out holes for the vents (circular viv vents), silicone vents in (we used non-toxic aquarium stuff), sillicone double runners top and bottom and fit sliding doors. Since we took the pic below we added a strip of Perspex slotted in vertical runners right at the front so we could have thicker substrate. I've also got an aquarium monitor on the doors to keep an eye on the temp and that worried me a bit but everything has been fine.
We chose not to add vents I the ends because the kallax is hollow so we thought it would take away too much of the structural strength.
Anyway, our hams love it and the amount on ventilation we have put in seems to be fine
Good luck with your project!