Well, if there's a point to this thread, I guess that's it- that it's possible to (hopefully) make something nice out of discarded old junk, using a bare minimum of more or less obsolete handtools.
Used the more or less finalised outer plate outline as a template for filing the inner plate. Shown on a 50mm square, FWIW:
Thanks DavidT, ajm, Bryncrug72, Hillwalker... but watch me squirm when it comes time to locate a Suntour-flavoured pulley-bolt! I spotted two likely looking stainless steel bolts in the roadkill zone just a few hours ago, as it happens. At least, I think they were stainless steel. However, I am a firm believer in subjecting gift horses to a thorough oral examination, so those bolts can stay there in their gutter until I am convinced of their a) availability and b) oxidisation resistance. I don't just pick things up off the street you know! If some other organism gets to them first, then their need is patently greater than mine....
I will be performing the 'ritual of the sore black fingers' at some point, if all goes well- you know? After that, they can anodise themselves... or not..
I thought it was about time I fished out my Campag. NR to check out the cage dimensions. It's wise to work from a good model, and a wiser person than me would have had it to hand before they started..
In the picture, a measurement taken from the width of the NR cage 'ribs'. I will work down my cage plates to the same dimension, not that they're far off ATM- less than half a mm.
I also did some more filing on the bottom pivot casting- mainly on the back of it, getting into all the internal angles and rounding them off. This is not merely cosmetic- it will make it easier to keep clean.