I don't really want to spend the money for a Caldera Cone, especially since it's only good for one pot size. So I think I'll make my own.
Googling got me to a good paper pattern for cutting one out. See the thread here:
There's a bit of computer homework involved, but it's not too hard. You can print out a paper pattern after you input the dimensions for your stove & pot. So far I haven't got a full size pattern printed yet--still have to deal with translating from European paper sizes to my printer. But that's part of the diy-ness of this project.
I plan to make what the pattern-program-writer calls a "Caldera Clone" for my old 2-qt pot and one for the AGG 3 cup pot I'm getting.
There's another method here:
Have fun with them.