Yeah, but why bother.  You can achieve the same result using a pot with a lid and either another pot or a heat resistant plate that will fit inside.  Anything else you need, you can pick up as you go.  You may or may not need a "plastic" bag, depending on what you are cooking.  Your "meal" will be fully cooked, just not browned as in a conventional oven scenario.

This method is known as steam vat cooking and has been around for at least 20 years that I know of.  I've used it on both backpacking and canoe trips.

Just find yourself, 3 or more smallish flat stones/rocks (usually readily available on backcountry trips).  Rinse them to remove any debris.  Place them in the bottom of your larger pot (the one with the lid).  Arrange them in such a way that they will keep the pot/plate to contain the cooking item above the surface of the larger pot's bottom ... in other words, the rocks are spacers.  Add water until the rocks are nearly submerged.  Carefully place the smaller pot or plate containing the item to be cooked on top of the rocks.  Place on the heat source (camp stove or fire) and wait.  You can start your timer when you see steam start to exit the top.  Check periodically to make sure you still have steam and haven't lost all of your water.  Add more water as necessary.

Technically you can't burn your food with this method, as as long as you have steam and you can't dry it out.

So, where does the "plastic" bag come it.  It is used both to contain your item to be cooked and limit the additional moisture added to the item.  Personally, I prefer "boil-n-bags" for this use because they are a bit more heat tolerant should your bag touch the interior of the pot at a point where live flame is licking it.

Personally, I find the bag somewhat messy when serving time rolls around, plus it is just one more thing to pack and keep track of.  If your smaller pot has a reasonably tight fitting lid, you've got the same type of cooking vessel as the plastic bag.  Although I must admit, having steaming hot tortillas coming out of a plastic bag is hard to pass up.

Oh, by the way ... you want to actually bake something and have it browned (e.g. chocolate cake, bread, etc)?  Just turn your larger pot upside down and let the lid become your base.  Place the stones on the lid (as spacers), put your "cake" pan on top of the stones and then place the pot (face down of course) over the other items.  The pot is now your oven and will build up heat from the top down as it sets.  Without water in the arrangement, its pretty much like your oven at home.  Just resist the urge to peek before cooking time is up ... you'll loose all your heat and push the cooking time out longer.  You might choose to put a conducting thermometer on the pot (just lay it on top) as a way to judge internal temperature of your oven.

Traveling in the dessert or other places where you might not find suitable rocks ... you may be carrying "spacers" with you.  Check your repair kit and see it you have any larger hex nuts in there.  They weigh a bit, so if I know I'm going to be in that kind of terrain, I'll usually grab something like a length of metal conduit and lop off a few 1" sections ... just cut them square so they will stand up.

dd