From time to time we manufacture gates & railings which have a bow top similar to my attached sketch. These have a flat steel frame with round bars passing through the top bowed section which means that the holes are elliptical. In the past we have always "fudged" these as necessary but we are just purchasing a CNC plasma machine on which I would like to cut them.

I have two questions on which I would appreciate some advice. Firstly, what is the best way to sketch the actual bowed shape to start with and secondly how do I produce a flat pattern of it with the slots in ready to be exported to our plasma cutter, which would be in DXF format.

Incidentally, the actual profile of the bowed section is not critical, it just needs to look right.