Use a cheap cast‑iron pot for the fire, then put a smaller metal saucepan inside that pot like a makeshift double boiler. The heat from the pot will rise and steam in the small pot, so the liquid stays below boiling. Keep the lid on the big pot so the steam doesn’t escape, and use a cheap glass thermometer in the small pot to keep an eye on the temp. If you’re feeling fancy, hang a metal sling over the pot so the small pan stays centered and doesn’t drift. Stir occasionally, but don’t over‑simmer—rosehips and quince are pretty forgiving. That’s the simplest way to get an even, steady heat without fancy gear.