I didn't add the second outer layer of mailing paper but look even without it, such sharp corners! I imagine with some brown or white mailing paper on the outside it would have quite a bit more structure.
Enough structure to hold a gooey half dozen cinnamon rolls, a modest portion of bar cookies or a delicious sized for 2-4 person coffee cake. No liquidy batters but a stiff batter I imagine would do just fine. And no having to pay too much for disposable pans.