Thinking Out Loud:
A plastic tub or pond liner?
Pros and Cons: Need to dig a bigger hole for the tub and then need to pack the dirt tightly around the tub (if not packed tightly, rain water can fall under the tub and raise it up from the ground). Tub can crack in cold weather and cause a leak. Liner requires that all side are perfectly level around the edge or else it can leak. Also needs underlayment or sand to cushion the liner.
Ponds should be two feet deep for fishes and plants.
Waterproof pump? Solar pump cannot run a biological filter that is needed for a live pond. Maybe purchase a separate electrical pump for the filter?
Filter? Some pumps come with a built in filter down fish waste. (Need to put in bacteria?)
1" layer of coarse sand at bottom of pond
1" of fish to 1 sqft of pond surface area. Feed fish once a day to minimize fish waste. Don't feed fish when water is under 55 degrees. Put fishes in 3-4 days after pond is running. Fish eats algae and mosquito larvae.
Plant - 30-50% of pond surface area, mix of marginal, oxygenators, and lilies. Plants can be planted directly into the gravel.
Maybe no fish and no plants. Just pebbles? Then I might not need a filter...