Can I create a part from an Assembly Sketch?

I have a large assy that I sent out to a vendor to have a notch machined. He was confused at all the lines that came in when he imported it into Mastercam.

I told him I would re send the file with just the notch information as well as outside features/lines so he can locate the part.

My plan was to make a sketch in the assembly, delete all the components, extrude the sketch and be on my way.

Well I cant extrude the sketch in an assembly and I cant (or dont know how) to turn the assy sketch into a part file?

I also tried to export the sketch into DXF (cant export DXF of an assy?)

Thanks, Chris

SolidworksSketching