Im building a 6v, 21m diameter dome, Im ordering the cover from china. I have had a bit of trouble trying to sort out the best way for it to be made. Im going to be using a company that builds inflatable marquees. Does anyone know how I can work out all the angles and the best shapes for the cover to be made from? Im finding it hard to explain to them and when ordering from china it needs to be right as sending things back isn't always a option.