We have steam baked in a ziplock many times, but wanted to find a way to bake a round, uniformly thick cake or muffin versus the "lump in a bag".
I was after a "cake pan" for my Caldera Cone but after a lot of looking and considering, I decided to try the cheap and simple route.
I cut off the cake pan's lip, then re-formed it, using a roll of duct tape and my 1.3 L pot, to make a custom deep dish pie plate. Then I repeated the process for a lid.
I trimmed both for evenness and appropriate depths. Then I re-worked the folds in the deep dish to make the top edge a little smaller, and did the same to the lid to make it a little bigger. This made getting the lid on and off much easier. (I later added a "foil tape" handle to the lid.)
After a little more tweaking, I got the pan and the lid to nest nicely in the bottom of the pot to facilitate protection and transport.
I mixed the batter, oiled the foil, added the batter, and put the lid on. I added ½" of water to the pot and floated the cake pan, with the lid in place.
I started the stove wide open to get the water boiling, then went to simmer mode by capping the stove with a slotted muffin tin to reduce the flame to a minimum.
20 minutes later I had "Garlic Cheese Bread".
And a little after that I had a "Mixed Berry Muffin".
(We also make a corn bread, add rehydrated refries, rehydrated salsa, and "stick cheddar" for a "Hearty Refries", as well as other "hearty" breads for trail snacks and dinner supplements.)
The pan and the lid add 20 grams to our kitchen kit.
Done in the shelter of a warm garage, each test used about 10 grams of 85% ethanol.