Looking for my 12585 hood I found one that looked right and will try it. It's the rectangular plastic one by KMZ and was on an Industar-61 on the FED-2.
The KMZ one is a nominal 42mm and the Summitar would take a 41·3mm slip-on, so I should be OK. I'll have to pad it out with a bit of self adhesive black felt; so I'll scrounge a used cassette and recycle the light trap...
Also, FWIW, I've a photocopy of the original Summitar leaflet in the collection and (A) the milled rim doesn't have the groove in it and (B) there's a lot of 5cm filters mentioned for it but no lens hood. That doesn't surprise me, you seldom see them mentioned in old literature.
Regards, David