https://www.instructables.com/id/Cheap-DIY-Hot-Tub-Jacuzzi/
It's a Jacuzzi, but the same idea. It'lll be cheaper to go with a stock tank or beat up old aquarium UNLESS you can get stuff from other people's trash. Old fridges, freezers and bathtubs make great ponds. Sometimes you can pick up HDPE tarps - you can use a clothes iron, some parchment paper, and a good bit of work to patch them and weld them together into something thick enough for a pondliner. I imagine a fiberglass shower would be good if you built a 2x4 frame to keep it from bowing out. For small ponds in the 5-50gal range, people throw out rubbermaid and similar HDPE tubs. The bigger ones need rope tied and glued around the rim, 2x4s screwed around the rim, or 2x4 crossbars screwed inside the rim to keep them from bowing out (use washers on the screws). Once whatever it is starts bowing out too much, it will fail, maybe quickly, maybe slowly, but it will develop cracks or tears at the corners that will spread until it is useless.
It's a Jacuzzi, but the same idea. It'lll be cheaper to go with a stock tank or beat up old aquarium UNLESS you can get stuff from other people's trash. Old fridges, freezers and bathtubs make great ponds. Sometimes you can pick up HDPE tarps - you can use a clothes iron, some parchment paper, and a good bit of work to patch them and weld them together into something thick enough for a pondliner. I imagine a fiberglass shower would be good if you built a 2x4 frame to keep it from bowing out. For small ponds in the 5-50gal range, people throw out rubbermaid and similar HDPE tubs. The bigger ones need rope tied and glued around the rim, 2x4s screwed around the rim, or 2x4 crossbars screwed inside the rim to keep them from bowing out (use washers on the screws). Once whatever it is starts bowing out too much, it will fail, maybe quickly, maybe slowly, but it will develop cracks or tears at the corners that will spread until it is useless.
