I machine cases like that all the time on my mill. Get an angle plate tall
enough that the whole case can fit on the upright part of the plate, mount
the plate on the table and clamp the box to the plate. www.use-enco.com or
similar suppliers sell angle plates pretty cheap if you don't mind the cheap
and probably radioactive chinese cast iron. If you have to do a lot of these
boxes, drill holes through the plate that line up with the holes for the
case cover, and instead of clamping the case screw it to the plate from the
backside.

I just use a 3/8" endmill to make a slot for d-sub connectors, as follows.

Drill the screw holes for the D-sub connector first. Then use a 3/8"
endmill, centered on the screw holes, and mill a slot up to about 0.050" of
the holes.

Low cost import d-sub "Greenlee" style punches are also available if you
shop around a little.

Good luck-

Paul T.