Since this has been asked several times ...I thought I would post a "heads up" ..heavystar has in stock 46mm wide and 55mm wide screw in metal hoods. Guy came up with this or was an early adopter .
The 35 lux and the 28 summicron both have large hoods and in the case of the 35Lux a madding attachment process. The screw in metal hoods from heavystar ...a ebay seller ...are great replacements. They are cheap less than $10 so if you dent one no big deal. I think they are best left on the lens without a cap. My standard procedure is to use one of the Leica lens cloths and just clean the filter everytime I mount a lens and before starting a shoot.
He also has a 55mm wide version which is not dramatically flared .so its like a 60-62 outer diameter ...this works great on the 24 but cuts off on the 21.
I also use heavystar hoods on my 90 apo and 50lux because the built in hoods are too short and a M8 can handle a longer hood....I think these were a 46mm normal and a 55mm normal . Simple round metal hoods.
Heavystar is frequently out of these so when they have them its best to get what you need. You can find them by looking for heavystar s ebay store .