There are a couple of things you can do in this situation. The easiest is to use a piece of aluminium foil. Just place the unwrapped camembert in the middle of a piece of foil, and bunch the foil up around the cheese to encase the sides. If you have a small oven dish, cast iron skillet or mini pie dish, you could use that too. You just want to put it in something because it will explode and ooze everywhere if you just put the cheese straight into the oven.