Electrical connection from wall to heater

Trying to figure out how the electrical run from the wall to the heater should be configured. I have THHN wires running from the controller into the sauna, but they need to be in conduit. Does it need to be metal conduit or is the plastic liquid tite type conduit okay since it’ll be below the heater and not subject to much heat (or will it)? Does it need to be waterproof, because most of the metallic flexible stuff isn’t. Wondering what others’ look like (pics welcome).