Before any design comments you are aware that racks really do better in rooms of constant temperature (80ºF) and Humidity (60% RH) !?

The design should be based on the space partially closed body racks tend to be a touch better in rooms that get cool open ones in spaces that get hot. Remember that any enclosure system is only as good as the thermostat controlling it. Personally I would recommend the Herpstat 1 as it has additional safety mechanisms built in that make the need of a fail safe questionable. Other wise I would very strongly recommend a fail safe thermostat as well.

Building a rack I would use Baltic birch plywood as a sides and back as well as bottom. The 'shelves' and top I would use PVCx for its moisture resistance and easier cleaning. In my region it is required to have secure enclosures and I think if I was building a rack from scratch I would include a locking system on one. Perhaps a drop down bar if it is not too may shelves or drop in pins (golf tees work well)

To be completely honest I would suggest you not build a rack and actually buy a purpose designed enclosure, like a animal plastics or vision. there are 100s of others around these are just two examples. They tend to hold heat and humidity better and offer much more flexibility of heating (rhp che lights UTH ... as apposed to racks UTH or nothing) Start with a single snake and give it time you may (likely will) encounter problems and multiple animals will just compound the issues. I would wait for a year and then once everything is dialled in then decide on a rack and more. It will give you the experience to make intelligent decisions.