I always keep a spare sealed up and velcroed to the bike in an inconspicuous spot somewhere but you could have a duplicate made on a flat blank and just keep it in your wallet.
I use the built-in clip in my 'Stich pocket, but the "behind the side-adjustable wing" option is a good one...that's easy to get to and unlikely to be known about by non-FJR folks.
Didn't used to have one, but did put one on the bike on a trip to Colorado, won't say where, but damn if I didn't lose my key the second day in Dillon. Go figure. Lots of places to put one though. I'd do it if I were you.
On a long solo ride once I couldn't find my key, days from home. One time when I thought "damn I'm smart!" as I pulled the spare out of my tankbag inner pocket. Found the key when I got home. Damn whiskey.....
On a lanyard round my neck, together with spare disc-lock key, security cable padlock key, and front door key - always hoping She doesn't change the locks when I'm away