It's greatt that you're putting them in such a large tank, they'll love it! You'll have to show us it when it's done

I'm not really knowledgable in how to make custom backgrounds, but you can search online, I believe there are a few tutorials on forums. The Exo terras backgrounds are pretty bad for the insects getting behind, so if you do a bio-active set up then I would not use them or you might need to tear the tank down to get dead insects out. There's cork and coco backgrounds that can be siliconed with a safe sealant for fish.