GP,
A buddy of mine made a kreisel tank (U-shape) over a huge BBQ in Africa (not too many shops around where he was). You just have to really keep an eye on the heat. I think he layed the piece out on some metal mesh and played with how high above the pit it was to control the heat.
If you really need to use your oven, You could also cut your pieces in half, heat them, mold to a half cone, then solvent them together. Not pretty but functional.