I have similar story.
I had a ten gal
I was trying to do what you're doing, and I was a noob at keeping fish and all.
So I went and found a few decent rocks intended for a 3d background. It was a success, I boiled the rocks, let them cool down and bought hot glue to glue the rocks together. I didn't know how I would do it so I cut up some peices of clear plastic (was enough to cover one portion of the tank) and use it as a base so I could lay the rocks together while I hot glue them. After I finished glueing the rocks, i was quite satisfied with how my DIY background turned out to be. Let it sit dry for about a day or so. Pretty much now my rock background is just hot glued rocks on a plastic sheet. Then I slipped it into my tank, it was heavier than I thought. So I set the tank up, rock bg went in first, sand, filter, heater and water. Had the tank run over night. And then, here it comes, prices of rocks started falling off and then it just kept coming. I was quite disappointed, but It turned out to be a fun project I did once dealing with 3d background. If I had more $ and tanks I would def. do another 3 d bg.
Ehhehee, t'was hard to read and understand what I posted. Sry for the confusion.