You can find a free hot tub/jacuzzi on Craigslist that needs rehab. Might be a good cost effective method. Just need one without cracks or leaks in the tub.

We had one for several years up in New England on Canada border. 240V, dual motor. Some winter Months our electrical bill was $200 or more than before we had a hot tub. It was a 6-8 person Spa. Ouch!

If you could engineer one to run off of a wood stove or other combustion based method, it might save you some Benjamins.