Hello and welcome to FF! You can mold a background out of pond expandable foam and cover it once shaped and dry. For cover Titebond 3 (waterproof) with sprinkled substrate works well.

The water feature does not go through the soil. You use a clay ball layer high enough to keep substrate above the "water table." A 10 gal. is quite small and will be challenging to include a water feature with filter/pump, etc. Good luck !