The cobbler in town wouldn't sell me any rubber soles to whittle new feet for my down at heel Remington Rand Model 1. Said he didn't want people going off mending their own shoes, even though I told him it was for a typewriter. Fair enough. But he did sell me a piece of 8mm flat rubber he uses for stacking soles for £2 - enough for 8 feet. Much better material than the heels would have been. It is dense enough to just be able to deform it a little when you pinch it. I used the metal 'shells' as templates, then cut pieces to size and shape with a Stanley knife, chamfering each of the edges and sandpapering to round the corners. For these to work, close is good enough. Clamping the shell to the new foot, I drilled a 4mm hole through to take the bolt. The photo shows the sequence.
The bolts have a shank which prevents them being screwed in too far and I'd evnvisaged having to cut a recess to accommodate the square washer but tightening the bolt snugged it well into the rubber. By the time I'd installed all four new feet, it was obvious that there was a small chance the bolt heads would scratch whatever surface the typewriter may be used on. There might have been 0.5mm clearance, and these will inevitably compress over time. If you could get this stuff in 10mm, it would be ideal. I just stuck on a handy patch of 3mm neoprene to cover the base of each foot. A bicycle inner tube repair patch would probably have been OK too.