Chuck backplate - how to make it?

Looks simple enough but it could be tricky getting it done without screwing up the runout or having to manually machine loads of stuff. Any cockup here will fuck the whole thing up. 

The backplate sits between the harmonic drive and the chuck. The chuck bolts to it with three M8 cap heads (I will drill these in the chuck body later). The backplate is fixed to the harmonic drive using a plethora of M5 screws - there are 16 holes in the drive in total but I will only be using about 12 of them, as the chuck fixings can't avoid all those positions. 

I have some nice looking 1/2" flat stock, 150mm wide, so that will be ideal. Not least as I have nothing remotely available as an alternative.

I'll need to finish the backplate on the lathe to get a decent (lack of) runout but it's hardly going to be a simple matter even to get to that point. I'm planning to do most of the work before it gets to the lathe:

• Machine the "front" features ie the central bore, myriad fixing holes (M5 clear) and their counterbores (M5 cap head), the M8 threads for the chuck fixings, the register for locating the chuck and the outline roughing, to liberate the result from the (rectangular) stock. Finish machine the central bore, so I can pick up on it later. 
• Then flip it over and rough out the register for the harmonic drive, picking up on the central bore. I could do this on the lathe - I'll see how I'm feeling at this point.
• Finish turn the harmonic drive register in the lathe, picking up the central bore from the milling operations.
• Finally, mount the harmonic drive in the 4-jaw chuck, dial it it carefully and machine the backplate register in position. That should do the trick.

Here's the full set of operations. It's not the simplest of tasks:

Positioning within the stock, to leave a bit at either side for the tabs.

This is what it reckons should remain. Held in place with 6 little tabs.

Let's set it up, once I've done my painting duties in the bathroom....