Hi Ken,
If I were to build a highland terrarium, this will be my design. I am basing on a 48"x24"x24 external tank with an internal 36"x20"x21 internal display tank.
The display capacity will be 330.3 litres.
The water volume will be approximately 137 litres/
Since the water capacity is small the glass required need not be thick and the chiller required need not be big too. The chiller will cost around RM800-900. The chiller will cool the water and hence the display tank to 20°C or less.
You can scale the tank size up or down. Of course the larger the better.
Choong