Not a diy, but we love our chofu.
https://islandhottub.com/wood-fired-heater/
Thought of diy-ing copper tubing around a firepit, but the location is too close to the house for me to be comfortable.
Chofu is a wood fired hot water heater. We use the prunings from the yard to heat the water to soak in the tub and make charcoal to throw in the chicken and goat pens to become inoculated as biochar to use in the garden. I have difficulty breathing any time I get into a chlorinated pool of water and it gives me itchy skin. So I was glad when Frazer Mann of Island Hot Tub educated us about Hydrogen Peroxide. With the Chofu, there is no motor noise (works by thermosiphon) or bubbles. Just a nice warm quiet peaceful soak under the stars. Our tub is just a deep bath tub I got off craigslist….Frazer was super helpful when educating us just where to cut the out and intake holes in the side of our tub. Very promptly responsive to any questions I sent by email. When we empty the tub, it runs into the swale that waters the front garden. Peroxide is safe for the yard (what is left of it by the time it leaves the tub).