Here you can find the solution to a simular problem.http://www.dslr.co.uk/x-pro_filter_on_12-24/index.htm The sigma 12-24 also has a bulbous front element. Solution seems a lot of work though. Connecting a polarizer will be a lot more difficult......maybe a polarizer as big as the inner diameter of the lenshood could work if you could squeeze it in place. Could be that its not wide enough at the 14mm end.
