I've occasionally wanted such a thing when I've put something heavy enough on the passenger seat to trigger the seat occupied sensor.
Have you considered using the existing seat belt? Just loop it under whatever you're putting on the passenger seat and click it in, problem solved.
That's what I did but something without a belt attached would have been easier.
I can imagine if I was something like a Hermes courier using my car to deliver parcels it might be a big enough nuisance that I'd buy a spare buckle.
@Greenbank started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
I've occasionally wanted such a thing when I've put something heavy enough on the passenger seat to trigger the seat occupied sensor.