Bought a discounted caldera cone (discontinued model) at BPL, then trimmed a little off the top to resize it for a barely larger pot.

Suggestions for building one: Start with a model cut from sturdy paper before cutting the real one from thin aluminum. Heavy scissors, but not as heavy as tin snips are good. Use a pen on the end of a no-stretch string as a drawing compass. A three dimensional cone is formed from a two dimensional (2D) circular band. Cone height is the distance between the two circular lines. The length needed on the two partial circles is about a third of each of the whole circles. For the top of the cone, 3.14 times the diameter of your pot is the 2D length needed, but add extra for an overlap to fasten the ends together.

Plan B: Get flashing tall enough, cut a length just over 3 times the diameter of the bottom of the cone. Form that into a circle with the sides slightly overlapping. Pin the overlaps together at the base, leaving the top loose. Then shape the top into a smaller circle that fits your pot. Mark where you need to trim, then disassemble to cut out your cone, leaving some overlap material on the sides for joining the two ends together.

The pro built one has a base that is 2 inches wider than the top, with sides 7 inches tall. Consider how tall you need your pot to be above the stove.

Since I haven't built one, if someone does, please extend the above suggestions into directions.