Depends on what you need it for. Personally, I'd probably get a 10' by 3' sheet of whatever, roll it into the appropriate cone shape by experimentation, fasten it in place and cut off the excess. But as others have said, more elegant solutions exist by applying a bit of geometry or just hunting down the formula.
Select as Best AnswerUndo Best Answer
That there stuff takes math and math makes my head hurt. I think lemonie has the right idea.
Try this one:unikatissima.de L
. Google it.