I'm currently using one of the 5.5-39 adapter from Heavystar without trouble on my Summicron-C (2598xxx, it has the "lip")
They are reasonably priced and work.
Since I don't have a Leica 12518 5.5 rubber hood and I use a filter (B+W 007 MRC) I have found a hood solution for it. A foldable, threadless, rubber hood made for the c-mount CCTV Fujian 35/1.7 lens, available on the bay if you search a bit.
This threadless rubber hood will slip away if mounted just around the 39mm filter but will hold in place if mounted around the 5.5-39 adapter, so it works.
So, my setup is: Summicron-C + 5.5-39 + 39mm filter + 41mm cap, and hood mounted around adapter.
The cap will reach the 39mm filter with the hood mounted.
A 40.5mm cap would be better with B+W filters, but I coudn't find any... so I put a bit of camera tape on the inside of the push-on 41mm cap. Other filters have a larger external diameter.
I described it here: https://www.flickr.com/groups/summic...7622716299533/
incl. other options I use.
Alt. I also use a plain 39mm filter partially screwed-in (with care)
I read around that it is preferable to have the filter as close to the front element as possible, but I'm not sure it's really important (in fact I'd prefer to avoid putting a piece of glass in front of a good lens...)
In case it's better, the best would be to partially screw a 39mm filter, or find a 5.5 clear filter and a 12518 rubber hood,
or a 11251 ring and the glass of a (37mm?) threaded filter (since 5.5 glass will not fit inside 11251) Note 11251 has no female threads on the top.
Some users lathe the outside of a 11251 ring to fit a 12526 hood. Or trim and sand the rubber of a deteriorated 12518 hood to do the same. But 12526 hoods are very expensive. I see here https://flic.kr/p/dQ56CC
that 12524 hood could also be used in this fashion.
The 12526 can also be used on the 28/2.8 ASPH Elmarit, so I guess one could find something more efficient for the 40/2.
I think the most elegant and safe solution if using a filter is Heavystar 5.5-39 adapter + 39mm filter + screw-in 39mm hood and cap.
Series 5.5 filters are old, most used, and very possibly non multicoated.
If you have/find a good condition 12518 hood and cap, that would be the most compact and simple.