What however was clear is that the feet design I made earlier wouldn't fit with the backplate. The quick fix was to stretch the top flanged diameter a mm to also take the backplate. (This won't take the extra thick washers that were on the machine.) The front feet base diameter section then also stretched a mm to keep it all 'on an even keel'.
The feet were then printed on an FDM machine in polyurethane rubber (Ninjaflex). Printed again by Martin's Hub via 3DHubs. He's got the machine settings for this material pretty much perfected by now and it prints very clean and to correct size. The printed feet snap right in the outer frame.
To create feet that will also take the washers to create some space between the backplate and bottom of the frame would take just some extra stretching of the CAD model. It does make the whole machine sit a bit higher. Judging the height of the machine from the rubber pads on the case base, it can't have been that much higher than this. That would make the original rear-feet design have a very thin base, almost have it sit on the case base. Oh well... :-)
The 3D model (Sketchup) for the adapted feet and the STL files are on Thingiverse here. If you've got one of these typewriters with dodgy or non-existant feet, highly recommend getting some new feet. For a machine without backplate, of course just manufacture a set of four 'front feet'.
(Another small mystery that remains is why the '29 backplate has a small hole at the rear-left of the machine. Surely the Remington Portable #2 doesn't have a shift-release prod?)