Hmm... It depends on the size but you could use ice. To do this
use a plastic bottle, the size or number of them would depend on
the volume of air and it's temp, that you want to keep cool.
Just place the bottle in the freezer in the evening then place it
in the terrarium at morning. Since ice takes up so heat in order
to melt you can get the temps too cold so don't alot at first.
Dave Evans