How to fill the gap between the background and the glass?

[DC101]

Hello everyone!

I have almost got everything to begin setting up my vivarium. I bought a very nice background that had a waterfall
with it but it wasn't exo terra brand, and doesn't fit my viv exactly. The background is a bit small.

So, i want to puts something between the top and sides so the crickets can't go behind and hide but i also can't use glue or
anything permanent because if the pump stops, then i need to take the background out.

So, any ideas?

Thanks!