IMO, melamine (melamine-coated particle board) likely the best wood material for a rack shelf. You can tape Flexwatt right down on it with no worries of warping, etc. To ignite melamine, the temps would have to be so high that the plastic box would first melt and your poor snakes would be in big trouble. In other words, worry about maintaining proper temps for the snakes, and don't worry about melamine catching fire. Flexwatt puts out relatively low heat compared to some products in your home, like toaster heat elements, hair driers, incandescent light fixtures, Dell laptops (lol).
The rack sides can be plywood or melamine. You can go with the open-rack design and use 2 side supports per rack side.
You can use wood strips (like 1x2s) or pieces of aluminum angle as the supports for the shelves so you don't have to screw into the sides of the melanine if you don't want to. Just make the shelf wide enough so the box below fits within the area between the shelf supports on the left and right side.
If you keep the lids on the boxes, the rack tolerances can be more relaxed.
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Mark
