Being basically lazy, I borrowed a DE9 (DB9, but that's not technically correct; a D sub miniature or D Sub connector) from a different design and used OpenSCAD to hack away the part I didn't want, as well as add an apron with mounting holes.

Very easy to print but you need to be sure your dimensions are accurate. The idea is you can cut a pretty crude rectangular hole in your panel big enough for the DE9 to pass, drill two holes in the panel, and then this plate covers up the ugly hole. The connector hangs on by the jack screws.
Note: to modify the OpenSCAD file, you will need the connector mount file from mrbluesky (note the "remixed from" to find the file).