The “Remove before flight” keychain I got from one of our workshops (I work for an airline). It was one of those “get togethers and know your airline partners” type of functions that gave out trinkets and such. I think that was one of the coolest thing I got out of that, aside from some pens (bleh), booklets/bookmarkers and some other stuff.
It’s actually pretty handy ‘cos I sometimes forget about keys. and it’s easy for me to pull out of my jacket or bag/pants/pockets instead of fumbling around trying ot feel for a particular key (when I have other keys in the pockets).
There are several aviation sites that sells them, with prices varying between 3USD up to 7USD. Although I’m not sure which one is best ‘cos I’m looking around myself trying to snag a few more for some friends.