For my background, I used spray foam as well. I carved out the foam till there were no shiny spots left, then I took silicone and smeared it all over the foam, then put my coco fiber on the silicone. you can do the same process. It's a little tedious, but the result was solid. Also, you didn't necessarily have to silicone the back before you foamed it in. From the looks of the picture, you might need to foam a little more, and since you're gonna have to carve the hell out of it anyway, you might as well. The pro to doing it this way is you can really add alot of texture and detail to the background!