A bain-marie is a double-walled oven pot with water in between the walls. This construction limits the maximum temperature to the boiling point of water, which is 212°F (100°C). Two suitably sized oven-proof pots can function as a bain-marie, provided that the inner pot can be kept from touching the outer pot (except perhaps at the top edge) and can be kept stable enough.
Oven temperature should be 325°F to 350°F (165°C to 175°C). Start the bain-marie with boiling water, instead of waiting for it to get hot in the oven. To prevent baking, use a lid and ensure that the water goes well up the side of the inner pot.
A bain-marie can be used to cook custard, preventing the outside from crusting before the inside is fully cooked.