Hey there! I know I'm late to your post here but I just got signed up on the forum. I'm doing something similar, but with orchids. What I'm currently growing would like to get down to the mid/high 50s at night (<80deg during the day) in the summer and probably about 50 (<75deg during the day) at night in the winter. I live in AZ and even though they call it the valley of the sun, they really should call it the surface of the sun. My electric bill in the summer prevents me from keeping my house lower than 80deg during the day and 74 at night. Sometimes it's so hot I can't even sleep... And I want to grow some other stuff that really should be down to the mid 40s and night in the winter...
My current terrarium can't handle the cold-growers as I struggle to get it down to the low 60's at night here in the summer. It's got an aquarium chiller going up to two, 240X120mm PC radiators with two fans each, both inside the terrarium because I can't do much outside the box (36"x36"x18" exo-terra.) To seal it well enough to keep the cool air in means I'm not getting enough air exchange at 80% humidity and I also struggle with fungal/bacterial infections on my plants. There's a lot of airflow in the box; 6 pc fans (4-120mm, 1-140mm, and 1-80mm, all operate 24/7.)
Due to the house temps, it doesn't make sense to add another AC to a room. It would have to run 24/7 and at least for these plants, 60 degrees isn't going to cut it. Some summer nights the outside temps don't get below 90, so that's really inefficient for me. The regular terrarium glass isn't very thermally insulated either.
So I'm upgrading. I'm building some terrariums out of wood (just 1/4 inch plywood with furring strips for structural support. Hope it holds together!) I will be making the doors the only parts that have glass, and I'll be making them double-pane. The Interior will have foam insulation bonded to the walls and the whole thing will be waterproofed on the inside. I'm taking inspiration from a DIY aquarist in Canada who gives tutorials on [among many things] building aquariums using wood.
It's a 4-part series and the terrarium project will be more of a cabinet than a terrarium, but that was my inspiration. And it doesn't need to be nearly as heavy-duty since it won't have the pressure of the water inside.
Since I'm struggling with the cooling, I'm actually going to go with a setup in this post: DIY Air Conditioner – Cooling Systems for Highland Nepenthes | EdenCPs
They basically modify a window-AC to chill a propylene-glycol mixture in an ice-chest, which is then pumped up to a heat exchanger that is ducted to the outside of a terrarium with an inline fan that kicks on via a day/night thermostat. There's another person on here who posted their setup from about a decade ago (couldn't find the thread to link to, but it's how I found the forum ironically,) who basically did the same thing but instead of a modified AC unit, they put their coolant reservoir in a chest freezer. I was going to do this instead BUT after reading their post thoroughly I noted that their terrarium was in their basement which stayed between 60&65 at night, and their cooler only got their terrarium down to 50 degrees so that's only a 10-15 temp drop for the terrarium's air mass. If that's all you need that may work just fine for you. I need a minimum 20deg temp drop at night in the summer, and almost a 30deg temp drop at night in the winter for what I'm intending to grow. And my reservoir needs to suck the heat out of 3 terrariums with a total of about 35 cubic feet (the third terrarium will be more intermediate and won't need such extreme temp drops, but it will still add heat to the reservoir and therefore add more of a load on the coolant/AC system.)
This is still all very experimental for myself, but I'm here to tell you that at least with my house temps, the aquarium chiller with PC radiators in an all-glass terrarium really doesn't cut it. I still think it's doable, and if you're much further north than me, you may be able to get away with just installing a window AC in your terrarium's room. If not, check out the link I shared above for the DIY air conditioner and think about whether or not that may work for you. A photo of my setup is attached. Like I said, it's barely cutting it for now, and won't be good long-term for the plants I'm growing, or especially insufficient for the cold growers (nepenthes included) I want to get this winter.