My aunt has used a hard-sided above ground pool as her main pond for going on 15 yrs now with no leaks. Her koi are at least that old and HUGE! You can find them fairly cheap at a big box stores. I think buying one to use as a temporary pond would be a pretty safe bet. It's also nice to have on hand in case of emergency (like quarrantine) or if you need to re-do something on the pond at a later date. She just runs a submersible pump to a UV (because in full sun), to a small biofall that falls into the pond. And the biofall is really just sitting on top of a drum. Not pretty but it does the job.