Stingray 4540: Ha ha! Thanks for fixing that. I was in a hurry getting ready to head out the door and I originally put "tin foil" then realized that that's an old and incorrect term and would date me, so I corrected it to aluminum foil, but left out the foil. See, I'm getting so old I'm forgetting! I'm actually fairly young, but I've probably hit my head too many times after jumping out of airplanes. Darn kevlar's need more padding!
I'm a big fan of aluminum foil. I have several older pieces of Japanese and Thai titanium pots and cups that came with stainless steel lids. Must have been harder to make the shaped lids or something back then, but the lids weighed as much or more than the pot or cup. I replaced the lids with doubled up aluminum foil and it was usually fine for a single expedition, although they're easily lost in high winds. When gram counting on expeditions I was counting I still replaced the titanium lids on my pots and the lids on my insulated cups, to save just a little weight. When you're carrying all the extra gear for clients you shave off any weight you can (cut off and drill out the toothbrush or use baby finger tip toothbrushes, cut off map edges, trim off tags [I've trimmed over a pound this way before], replace aluminum tent poles with custom carbon fiber poles, use down sleeping bags rated to much higher temps than you're going to experience and wear clothing and use hot water bottles, etc. etc. etc.).
For the bundt cake pan it would be easy to cover the whole pan in foil, cut an X into the center and pull it down tight over the chimney to get a tight seal. If you want heavier duty you could get the aluminum oven liners at dollar stores and grocery stores. They're much heavier duty and so much more durable, although less pliable so can be trickier to work with and when they get a kink in them they tend to be more permanent.
The aluminum flashing that Stingray mentioned would be a good alternative if you want something more durable, just make sure to round those edges so you don't get cut up.
I can't wait to see your results, so please post them up with lotsa pictures!
