I thought Rob meant that he couldn't shell it out like the original. This being a requirement for injection moulding more than any other reason. Rob shouldn't have any problem machining the recess in the back for the lens. That should be the easiest thing of all to machine. As for the draft angle on the button cutouts - well that would be a bit of an PITA without a cutter with the correct angle ground on it. You'd also have to machine the button cutouts from the back of the fascia. You can get die sinking cutters which are specially ground with angular sides for machining injection moulds. I don't know whether Rob's used these or not. I'd actually be very interested in details of Rob's setup, sequence of ops, clamping etc. for nothing more than to satisfy my own interest.

I'd probably have done it the following way:
Op 1
(1) Start with rectangular block of Ali a couple of mm longer & wider than the finished Fascia and about 3 - 4mm thicker (to allow for clamping in a vice).
(2) Machine Back Face flat with Face Mill. Machine pocket for Lens & outside profile of fascia with Slot Drill.
(3) Machine cutouts for Buttons & Knob using die-sinking cutter.

Op 2
(1) Mount part on fixture using 2 of the machined edges as datums. Use something like Mitee-Bites for clamping.
(2) Machine off excess mat'l used for Op 1 clamping using Face Mill.
(3) Rough out Main Windows through.
(4) Rough 3D contour.
(5) Finish 3D contour.
(6) Drill & C/Bore screw holes.
(7) Finish by hand.

There's obviously some detail missing from this layout but that's the general way I'd probably have done it.