For my BOB I have a SS cup that fits around my water filter bottle. It's a hybrid Sport Berkey. I just replaced the Nalgene bottle with a SS Kleen Kanteen bottle so in a pinch I can remove the filter and lid and use that to boil water. For my full backpack I have the GI cup and also other nestable pots and pans and for car camping there is also my Dutch oven which has to be the greatest thing ever if you don't have to lug it far.

However, like many of you I am seeking something collapsible , really small and light weight better for my on body kit (OBK?).

Right now I have a piece of very heavy duty aluminum foil from a very old air force survival kit which seemingly was designed to be able to be folded and then unfolded and used to boil water and then refolded - but I confess I have not tried it. I am concerned that it will spring a leak and AFAIK there is no way to patch it in the field.

As a kid (a very long time ago) I had a collapsible aluminum cup that was a series of ever smaller rings of aluminum that fit together and held water fairly well (at least it did according to my now failing memory). But even if i could now find something like it, I doubt it could be used for heating as the rings would expand at different rates and in any case is larger than what I am seeking.

But I was looking at the collapsible silicone cups. They say they cannot be used on direct flame, but I was wondering if they could be used inside a aluminum foil shield which would then not need to be water tight. But since silicone is used for pot holders, I am wondering if heat would transfer to the water.

If not slicone, what else might work within a aluminum foil wrapper? (I know it's possible to boil water in just a paper bag.)