After reading this thread (and posting), I fished out an old 58mm lens cap I had laying around (remnant of a Nikon lens sold a while ago).
It fits the lens hood of my Hexanon (which is, apparently, the same size of a Summilux 35/f1.4 even though it's only an f2). Guess where the lens cap is parked now?
As for the distance and focusing... I don't believe just carrying the camera around your neck will burn the curtains. It should be exposed, as joe said above, aiming to the sun, and motionless, like left inside a car, lens up looking at the sun. I walk with my camera and the lens uncapped. Of course, I close the aperture down and point downwards, but for me to damage the curtains I'd have to stand still, point the camera to the sun, and let the light work its power... for a while.
Some time ago, on a nice, sunny summer day, I took out both Leica-mount lenses (it was my 'cron and my Hexanon) and put them on the ground, on a piece of paper. I even aimed them at the sun... and still didn't burn the paper. Not even slightly warm. Mind you, both were wide open. If this happened to me while attempting to use the lens with "evil" purposes... 😕
I guess it won't take a second to wreck the shutter curtains. 😱
Life is full of paradoxes... 🙁