We have all had the problem of getting that perfect radius for a funnel on a ship or some other model. We try and follow the formers as much as possible but some times it just doesn't work out. Now some body may have though of this already and I'm pretty some has to of. Fimnd a metal or plastic object that mathces the radius you need on the inside of the curve and take a piece of card and soak it with PVA glue form the card over the radius and apply a clamp to hold the stock together. When the glue is dry the card will hold the radius to a large degree. It seems the parts has literally become a composite of card in a glue matrix. This is siminliar to the trick of applying PVA glue to the back side of hull plates to stiffen them.