After much consideration I have decided to abandon making my own 3D background.
I was not happy with the results and I do not want to attempt to do the same with the time and money involved.
I purchased this today:
https://www.universalrocks.com/fish/backgrounds/rocky-style/various-sizes and I hope to receive it before next weekend but I doubt it.
Originally I had looked at a similar product on their page but was concerned about the price and the volume of water that would be out of circulation. The version I purchased is much cheaper and very flat on the back side so mounting flush to back side of tank will be a breeze.
Also purchased Seachem Stability today for setting up water in the new tank and a decent piece of driftwood. I still plan on another larger piece of driftwood...I shudder at the price.
Type of substrate is still a major debate in my head.
Read lots of pros and cons about gravel and sand.
Gravel: Harder to clean with siphon, food particles fall down in it and rot etc. Gold fish can actually choke on it. Not cheap.
Sand: Easier to clean with siphon (just sweep across surface) with waste remains on top of sand layer. But can get in goldfish gills and cause damage/irritation. Can be incredibly cheap. Avoid getting it deeper then 2 inches.