buy one, cheaper than fab one... thought it is more fun to fab one.... I just can't find sheet metal 16 ga or 14 ga at cheaper price than just buying a premade. or a roller.
Few people reported those Cheaper ebay Chinese ones works fine. at $20.
If you wanted to fab one, there is that cone calculator (it's a full functional shareware) that prints out patterns for you... that you can just tape those 8.5x11 paper together to make a nice cone cut out.