If you got your Exo-Terra new and it came with the foam background in it, I found they do make 18x18 background pieces for the sides too.

They are Exo-Terra part #Pt2955

http://www.exo-terra.com/en/products/background.php

They are hard to find but I have picked up a couple, That will help insulated the glass from the inside with out taking up a lot of enclosure space

Might just give you enough extra being you replaced the screen top too.

https://www.amazon.com/Exo-Terra-Bac...8&sr=8-31&th=1

I have never purchased anything from these places but here are some it looks like

https://www.aquariumspareparts.com.a...8-inch-pt2955/

https://www.petswarehouse.com/exo-te...nx18in-pt2955/