Wood fired is definitely the way to go for simplicity. Especially if you have a lot of wood. The exterior coil ones have two issues. The burn is inefficient which generates a lot of smoke. And the heat transfer is inefficient. Our submersible stoves will heat your tub much much faster as a result. Here is the diy package I put on the #takemysats store for just this reason. You can do something simple w a stock tank or do something more complicated. https://www.takemysats.com/snorkel/products/hillbilly-hot-tub-diy-package