You can either drill holes in the tubs and have it fit snuggly in between the plywood (when you measure for the tubes between the shelves make sure u use about 4 old cd's to place at each corner of the tub... in doing this it will give you just enough space so that the tub will slide in and out easy but keep your snakes in) or drill no holes and use the pegboard WITH the holes instead of the plywood. (you may not want to spend the extra cash for this though. I wouldn't but I'm throwing it in there anyway) I saw someone do this on the internet and they said they didn't have a problem keeping humidity in.
As far as heat goes IMO I would shoot for belly heat 3-4" flexwatt found at herpsupplies.com, reptilebasics, etc... I would watch a video on youtube if I were you so you can see the proper way to install it. Hope I helped. Good luck!!!