never done it, but my first thought for a cheap somewhat easy solution:
make a piece of steel in the shape you need, file to fit, drill the 2 side holes. harden this fixture. after this, just simply drill the 2 side holes (you can use the jig to locate them) then use those holes o locate the fixture and remove what is left inside the d-sub area. you can slam your drill bit in there, file to the edge, whatever, no need to be neat and precise, the hardened jig will stop you from making the hole too big. just get in there and hog it out.