It’s not the easiest job, but building your own wood-fired hot tub can save you thousands of dollars if you’re willing to dig a hole and learn some basic concrete skills. Use a shovel and a pickax to dig a pit, making it at least six inches bigger on all sides than the size of the finished tub you want. Be sure to pick a location free of roots or bedrock. Make the hole as deep as you like, but smaller tubs will heat faster. As for the shape, a keyhole design is ideal, since you can place the woodstove safely away from the larger sitting area. Remember to cut benches and steps, too.
Line the pit with plastic sheeting, using roofing nails to tack it into the dirt. Cut sections of two-inch-thick closed-cell insulating foam—available at any home-improvement store—and line the pit, laying the foam on top of the plastic sheet. Hold it in place using chicken wire, which should cover the interior of the tub and anything else you want to make out of concrete.
Get a professional to help you estimate how much concrete you’ll need. Mix the concrete in a wheelbarrow, slathering it onto the chicken wire as smoothly as possible. Start high and work low, since drops will fall to the floor of the tub. Wear gloves. Smooth? Good. Now let it cure for a week. Once it’s cured, paint at least two coats of concrete sealer. (Note: You’ll need to reseal it every year with concrete paint.)
Buy a submergible stove for around $750 (http://snorkel.com) and place it in the narrow part of the keyhole pit. You’ll want to install wood fencing between the stove area and the soaking area to keep people from accidentally bumping into the burner. Though the stove comes with mounting brackets, it’s best to anchor it with dumbbells to keep it from floating.
You’ll need a way to drain the tub. The simplest method is to buy a Quick Drain system—a handheld pump that siphons and vacuums out water ($75; http://hottubessentials.com).
Make an insulating cover out of two-inch closed-cell foam to hold in heat. Fill the tub and stoke the fire at least six hours before your party. The first heating can take closer to 10 hours, depending on the size of the tub. But once the water is warm, it can take as little as six hours to get hot again.
Aaron Huey made a giant wood-fired hot tub a few years ago for some of the best parties that northern New Mexico has ever seen. They involved numerous DJs, a giant golden bull made of fuel-soaked papier-mâché (which burned gloriously), and dozens of naked women.