What material is used on the arrowed cover seal?

it appears to be rubber by the way it deteriorates and most likely you can get replacement from MKO
I recently replaced mine with a moulded rubber strip obtained through Clark rubber. Don't have the exact measurement handy but is was approx 40mm wide with ends that fold around about 5mm kind of like a wide flattened u shape and cheap as chips too
It fitted perfect and looks to be a good dust seal. Apart from cleaning off the 50 odd year old perished rubber the hardest part was refitting using the rivet like pin that you have to drive into the alloy case. I can probably send you an email with a photo if my description isn't understandable

I sikaflex them on...

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did I tell you that I won a trophy?

Maybe a cut up inner tube will fill the bill.