I use electrical tape and then you can wipe it off after each shooting session.

I finally broke down and bought a Butler Creek scope protector.. what a piece of junk. It was on sale and would have costed me more to send it back to them then I paid for it. First time I tried tp put it on the scope, the strap tore. Not only that, it must have been made for a very small scope as it hardly reaches around some of mine...