[Nuttyman54]

As far as i'm aware, it is mathematically impossible to create a true flat helix out of a sheet of metal. Some bending/stretching may have to occur. It could work to use semi- or quater-circle sections and weld them together at the correct angles. Alternately, you can probably just use strips of metal on the underside to support some cloth or another sheer material.

And yes, the diameters of the cutout will be larger on the flat material than the diameter of the true helix. It's possible to obtain the measurement using the pythagorean theorm. I can post a pic of how to do this if you can't figure it out.