Here are quick specs on the cone kiln if you want to try making your own: It was made from a 4x8 sheet of 14 16 gauge steel with 3/8" rod welded around top and bottom rims for stability. Top diameter is 43". Bottom diameter is 16" and height is 16". It is open on the bottom so you just roll it away when you are done. That's it. My welder had a heck of a time bending the steel to make the seam weld. Needs a couple of strong guys or preferentially a differential slip roller to roll the form into shape. You could also make it from lighter weight steel.
Here is a paper that looks at biochar characteristics produced by this type of kiln. And here is a marketplace where you can buy one of the Japanese kilns. They are stainless steel, I believe and they come in different sizes. The 100 cm kiln is very close to the size of my version.