A regular modern Leica clip on cap- like for the Summicron 50 will go right on fine, and the hood/cap for the new collapsable 50/2.8 apparently works well also, though I haven't tried this combo yet. There is a ring that will hold a series 5.5 filter and thread properly onto this lens, rumor has it a 39mm filter removed from the mount will fit in this ring also, again untried by me. I'm leaning towards the ring and getting a pair of 5.5 filters for mine- my two most used. Right now I have a regular 39mm filter snugged into place, and this has worked just fine- even with the metal Rollei hood clamped onto the filter I've not had any accidents. Not the snuggest of fits, but quite serviceable. A regular 39mm spring type cap rides on the filter also.
Don't let the threads prevent you from going for the Summicron, it is a beautifully creamy image maker.