The empty 65 gallon Hex is calling to me, and I'd like to get started on it soon. The only thing that is holding me back is deciding what kind of material to use for the background. It'll be eggcrate for structural support and spray foam over that. I have a ton of elm bark set aside and that will be used on most of the straight areas and there will be pockets for live plants. However, I'm really having a hard time figuring out what to use to use as a base cover to naturalize the foam.

The tank is for a native tree frog and will be kept in my critter room, so no real worries about high humidity, running water or heating.

When I made the current 15 gallon hex, I used peat moss and concrete binder, and it cracked like crazy when it dried. So I patched that with peat moss and Titebond III, and that cracked! Those cracks were then patched with silicon and peat moss and doesn't look that great. The tank was usable at that point, the worst looking areas I was able to wedge in elm bark to cover, and the frog needed a tank to move into so I let it be even though I'm not thrilled with it. From a distance it's not noticeable, but up close (ie, staring at the frog or while feeding) the cracks are obvious. I had even let the spray foam dry for a week first to try and prevent any problems.

With the size of the new build I want it to come out looking as close to perfect as possible without having to continually patch it. Any advice on how to get a crack-free background this time? I've looked at almost all the options out there, but they've mainly been used with high humidity or water features, which I think prevents the background from drying and cracking in the first place.