With the heat pads, it would probably be easiest to run a strip of it along one end of the shelves or to one side of the back of the shelves, eliminating direct contact with any of the snakes. To do it for each level, you'd need to build a mini-shelf of sorts to separate the heat tape from the animal below it. In other words, something just large enough to allow the heat tape in and contain it, but still permit some air flow. There is also a form of plastic or pvc peg board that is sold that the heat tape can be placed between that and the bottom of the shelf and the peg board screwed into place - allowing ventilation of the heat tape, but preventing snakes from getting directly to it. In all events of using heat tape, be sure to place it on a reliable thermostat to prevent potential burns.
As for the plexi glass, there's at least a couple of ways to make doors with it. For hinged doors, you can carefully drill the plexi to be fitted with hinges. Plexi needs to be drilled first, slowly and with minimal pressure. Too much pressure or too much heat from the bit will cause the plexi to crack or starburst around the hole you're drilling. I've found it helpful to lay the plexi on scrap lumber and just take my time to make the holes, drilling slowly with even but gentle pressure. The hinges can then be attached with rivets, as can a hasp or clasp to secure the door. You can also build a door frame, attaching hinges and clasps to it with screws, and then screwing the plexi to it (which is what I've done and would probably be the easier option) or making channels in the wood like a window frame. You can also make a sliding door by using ready-made channels sold for the plexi or making the sliding channels from wood - either would work. However, most presold channels are plastic, so I would recommend screwing two small strips of wood to the shelf to the outside of the channels to prevent the snakes from pushing the plexi out of the plastic channels and possibly escaping (yeah, I can be paranoid LOL).
As for cutting the plexi, you can either have whoever you purchase it from cut it (if they do that where you buy it) or you can cut it at home. For home cutting, you need to make sure you have the measurements marked on the plexi. You'll need a good straight edge (preferably a square or something of that nature, I like metal straight edges, but a good straight board will work) and a sharp utility knife. Using your straight edge, score the plexi deeply with the utility knife along the lines you've marked for the doors (try to keep it in one perfect line, multiple scores will cause an uneven break). Once the scores are made (they do need to be pretty deep for thick plexi), place the scored mark over the edge of a table (making sure the scored mark is aligned with the edge of the table facing up) and gently bend the plexi. It should break evenly down the score. If it doesn't break evenly, you can usually use a pair of pliers to break off any little points (providing there is a straight score behind it). Using channel locks or other pliers with a flat nose, align the nose with the score and bend - it should break clean. You usually won't have extra bits unless the score isn't straight or you've scored in multiple spots (for example, in the process of making a deep score you run over the score multiple times and not all passes are in the same line).
It sounds complicated and it can take a little practice in working with plexi, but it's really pretty easy once you get the hang of it. *smile*
Raven