If one were to fit a centre main strap, the two bolt variety, what is required and what material spec should be used?
As far as I know the operation goes as follows;
-Mill center main cap flat
-Fabricate strap
-Fit with longer high tensile bolts

Is that it?

That is the general way people do it, but I'd consider the following:
1. Milling the cap flat weakens it to begin with, it is much thinner cross section when milled flat.
2. Commercially available straps are just mild steel or maybe EN8, nothing special.
3. There is no demonstrated need for it, many Minis race competitively with no strap. Centre caps only break when you downshift at a zillion rpm and use the engine for braking. That's what brakes are for. For the road, they are overkill.

If you still want to do it, I'd make one like my 1412 stroker one (I made it when I reckoned Vizard was god, and before GR pointed out that they are pretty much a waste of time).
I didn't mill the cap flat- I just surfaced it to clean up, then made a stepped strap out of EN25 (tuff stuff). Note also it's a bit thicker than the square ones that Mini shops sell you.
It has about .0005" (1/2 a thou) preload when bolted up, so it actually supports the middle of the cap...
Bolts are Grade 8.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

As doc says: Don't jump into second at 8000rpm from 4th and you won't need a center main strap

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Lillee - 1969 Morris Mini K