I got an e-tag for the bike, and had to mull over where to keep it. My pocket wasn't an option as it more than likely meant a stop to rummage for the e-tag. So I opted for a mobile phone cover with a belt clip on the back. Its an old cover for a fat phone, but it has a clear plastic front and I just clip it to a strap on the arm of my coat. Seems to work a treat. The only drawback is that it isn't entirely waterproof. What do the rest of you do?