In an effort to avoid the problem I have put a 'tag' of electrical tape sticking out the side of the cap. The idea is that you will see it when you look thru the viewfinder. Only proviso is the cap has to be put back on so it lines up with the viewfider. I do find however, that it helps me notice the cap even before sighting up and especially as I adjust the f stop
Same as Jim, I've been leaving caps on rangefinders for 48 years. I put red tape around the edge of the cap, and then instead of complaining that the lens barrel protrudes into my viewfinder frame, most of the time I see my little red warning that the cap is still on the lens.
I have a voigtlander, which is OK as David pointed out the meter doesnt work. My big problem is my Mamiya 6 where the meter works fine. I like Jeffs idea but the lens hood wouldnt let me see the edge of the cap. NOt keen about leaving the cap off as I dont like cleaning the front element much. Maybe I'll stick a note to myself on the top of the camera, might help.