Edited

Hot tubs are made of fiberglass or plastic and last almost forever if cared for. There are so many old ones out there you can pick one up for free or real cheap if you look. the hardest part is getting it home. Hook it up to a solar hot water panel and you can heat it for a few dollars a month. I have one set up like that. And you can build yourself a solar sauna that hits 200 degreees for about 30$ too.