VERY impressive work! It's lovely, and I'm sure they both love the new accommodations!

I'm waaay late to this thread, I know, but there's a couple things I want to mention.

First, good call turning it around so the floor is 2x2 ft! You must remember that corn snakes get significantly larger than garter snakes, and, likewise, need more space. The 24x24" floor is down there at what's considered "bare minimum" for a fully grown adult corn snake, but it works and gives them "enough" space. 24x18" would have warranted a later upgrade in enclosure size, as those dimensions would be simply too small to accommodate a corn snake. (Speaking from experience here--after owning a total 18 corn snakes, I can say with confidence that anything less than a 20L (30x12") or a similarly dimensioned 24x24" enclosure would be inadequate, and even those sizes can be quite cramped to the right snake, which is why they're the "minimums" and not the "recommended"!)

Could I ask why you're going to install fans? It's not a necessary move, and may only stress the snakes out (constant "wind") and cool down the enclosure too much... I could see hooking fans into a good thermostat set to "cooling" so they kick on when things get too warm, but I somehow doubt that's what the point of them is? There should be plenty of air exchange through the screen tops, the drilled holes in the side, and what airflow is coming in through the cracks around the door.

On the note of the UTH, the reason their packaging very often says to NOT use on plastic is because the manufacturers know how hot they'll get!!! And, trust me on this one, they DO get hot enough to KILL your snakes!!! I have personally measured assorted ZooMed heat pads, the most commonly used UTH on the market, at anywhere from 140-160F, after only being run 1 to 12 hours, respectively, and both measurements were even taken in substrate-free environments (pre-set up, and in a bare floor roach tub), so without the heat-retaining properties of substrate... So, with the layer of substrate, you're looking at the potential of these heat pads cooking your snakes at 200*F. Considering Corn snakes need only a "basking" temperature of 85*F, and that temperatures over 90*F can be dangerous to them, that's easily more than enough to kill them.

To keep heat pads from getting dangerously hot, you need some sort of controlling device. For a very cheap fix in a very stable-temperature room/habitat (something yours are not with heat lamps on just during the day), rheostats (fancy name for a lamp dimmer) can be used. However, these need to be constantly watched and adjusted, using accurate digital thermometers (probes placed UNDER the substrate, directly on top of the glass--or, acrylic--above the heat pad) to be sure it's not getting too hot. The other, much more preferable alternative is a thermostat. Thermostats have a probe which you place directly on the heat pad (like you would a digital thermometer). You set the desired temperature on the dial, and it will turn the heat pad on and off as necessary to keep the heat pad only at the desired temperature.

If your heat pad is properly controlled, there is no risk of the acrylic, plastic, or whatever other material melting or otherwise warping. After all, if it were to melt/warp at 85*F, you would never be able to use the material in a region that has a warm day! If you look up the melting temperatures of plastic, you'll find every product used for animal enclosures, storage/food tubs, etc has a melting point of over 200*C (something like 400F?).

What I'm saying, to the OP as well as anyone else who reads this and was not already aware of the serious risks, is if you don't already have a thermostat for every heat pad, get one!!! It's direly important. Your options are many, keeping in mind a thermostat's price usually reflects its quality... While a $30 ZooMed Repti-Temp 500R is known to fail within the first year of use, a $100 Herpstat or similar, high-end proportional thermostat may last through years and years of use without a single malfunction (not that you shouldn't still monitor them, as thermostat failures can still happen no matter what make/model thermostat you're using).

Also, Nick, the only heat source your corn snakes need is the UTH.. They are very cold-tolerant and tend toward very terrestrial (not saying they don't climb, as they do; they just don't spend significant amounts of time, or, specifically, their digestion periods strictly in the trees). They are also a crepuscular/nocturnal species, meaning they don't naturally come out to "bask" in the sunlight, rather collect all their heat energy from the sun-heated nature around them after the sun goes down. Though you can use a heat lamp if you feel the need, all they really do is dry out the habitat and put your snakes at risk of overheating if the room gets too warm and the habitat is too small for the snake to escape to a cooler place (part of why heat pads are preferable, since they only heat a portion of the floor). I personally only use a low wattage fluorescent tube to light my snakes' habitats (keeping in mind I only keep crepuscular/nocturnal species at the moment), if I choose to light them. I've actually found my corn snake is more likely to come out and lounge around in the open if there's no bright light source... (Can't say for other corns, as I never used lighting on any of my other corns when I had them.)

Now that all of that is said... I apologize to members that don't use Fahrenheit! In my typical American-ness, I still don't know Celsius conversions...