You might want to check on the new CV 50/2, but I'm not sure that is small enough. If you want small, look at the CV 35/2.5 Pancake, with an LTM to M adapter you are set with a very small kit.
Look in a good hardware store that has a large selection of O-rings. Prexes (not sure about the name, it's been years) might have something. Put them at the front of part of your lens barrel and they work great. You might need two, depending upon the width of them.
With the right O-rings, just about any of the 50s will work. Somewhere on the internet I remember someone giving the max depth for a M5. You might check on the CL too as I think they share the same design. As long as you are a few mm away from the arm you should be fine.
The more I think about it do some searching on the CL too.
I'm not a 50 guy, sorry.
B2 (;->