Sauna Hourglass Timer

Leave the phone outside. This sand timer runs for 15 minutes with a mark every 5, so you can pace your rounds by eye, then flip it on its wall pivot to go again. No batteries, no screen, just thermowood and glass. It's rated up to 80 °C, so mount it lower on the wall, at least 2 m from the heater and the door, and out of the way of splashing water.