After owning my Slingshot for 2-3/4 years, a couple weeks ago I left my gas cap on the rear deck and lost it after I drove away from the gas station. I used @samowens44 idea and initially used some Velcro to secure the cap against a roll bar, but I was concerned it might not stay secured if I left the gas cap there and hit a decent speed so I thought I'd try making a tether to secure the gas cap.
My first thought was to try and make some kind of rotating plate that I could secure against the bottom of the gas cap via a hose clamp or by gluing some blocks tot eh gas cap, but I wasn't sure what glue to try and the hose clamp looked like it would stick out too far. I also wasn't looking forward to trying to cut a large washer-style piece with a lip to secure some kind of chain between the gas cap and the gas filler neck and making it smooth enough to freely rotate around the gas cap, so I found some light chain at my local True Value.
Step 1 - Step 2 -
Here's a GIF showing how the tether ring should rotate around the underside of the gas cap -
Step 3 - Step 4 -
I still think a rotating metal disc held in place somehow against the underside of the gas cap with a pull chain or slightly heavier 'bathtub' chain might work better. As long as the metal disc can rotate freely, a shorter pull chain/'bathtub' chain tether could be used w/o getting caught between the filler neck and the nut that secures the filler neck/tank to the rear deck insert. The cable tie ring worked as an expedient way of holding the tether ring in place against the underside of the gas cap, but a more secure method would obviously be better. I was reluctant to drill thru the side of the gas cap since the gas cap seals the tank in a non-venting manner and drilling a hole might compromise the seal.
I challenge our more creative members to devise and sell me a better tether mechanism.