I have come up with a creative solution to commercial bellmouths! I found some little tapered alloy dessert tins (for creme brullee and the like) in a catering supply store that have a base diameter of 43mm then taper out to around 70mm. Cut the base off and clamp away! They are around 3 bucks each and come in heaps of different sizes and are also available in spun alloy. Nice, I love creative solutions that cost 3 bucks. Cheers, Ben.