So what can we DO about these two problems:

• Takes a long time to get hot
• Costs too much to keep hot

Easy.

• Hack-in a better heater
• Hack-in a cheaper heater
• Add a ton of insulation.

To be clear, this isn't all original, clean-sheet work. The idea of using a propane camp shower on a portable hot tub has been done before:

https://www.instructables.com/id/Portable-Water-Heater-for-a-Hot-Tub-or-Pool/

https://www.instructables.com/id/Portable-Hot-Tub/

This one even went so far as a solar heater

https://www.instructables.com/id/Overview-2/

Excellent idea, and one I'd like to try, but at this point in my case it seems more cost than it's worth. And I don't want to buy and keep an extension ladder, in a rental, just for this. I could borrow one, but only if I had a VERY tolerant extension-ladder owning friend who didn't mind not seeing it again for 3 months of on-off work.

Anyway, while it's not all original work, I do believe I'm the first to have taken the idea to such extremes as IoT integration and closed loop control. More on that next.