There are lots of options for "nesting", but for me personally, a metal cup always seems to limit me somehow. I've seen a lot of Breeden's kits based on a Nalgene and holder, which seems like the best way to go. But I just don't want to have that bulk on me.
I'm going to work on my aluminum foil folding skills, and perhaps move to a smaller pelican case. But my mind keeps coming back to wanting a metal cup. Maybe a Guyot bottle as a base? I think I'm slightly stuck in the wanting a mini-kit to have as a backup, and a really good minimalistic base kit that I use.
And of course we can allways fall back on the folded bread pan. Or very heavey duty tin foil. I did a review a long time ago about using run of the mill stuff, and I found no matter what, as soon as its folded up, it gets holes in it.