I use the bag cooking method all the time.  It works for just about any kind of dough, except brownies - not sure why.   Muffins, cakes and corn bread are what I usually do.     The down side is it does take a bit of gas.   But chocolate cake is always worth it.

I have a set of stackable pots/lids.  I take the largest pot put about 1.5 inches of water, then put the lid for the next size pot in the bottom of the pan, right side up.  Then you have to use a 1 gallon heavy duty freezer bag.  It cannot be the thin kind.    I use the mixes that only require water, and I just put them right in the bag I will cook them in before I go, to save space.    Then I just add the water, and stick them in the pot of boiling water.

Then I put on the lid, with a substantial rock on the top, so it sort of works like pressure cooker to raise the inside temp, and helps keep the pot from running out of water.   In about 15 minutes, I take the bag out, dump it in a fry pan to brown and dry it out a bit, and you are all set.   Occasionally the bag will get a hole in it, but as long as you do not have a ton of water in the pot, it does not seem to really matter much.