Pastorbottle
·You haven't seen these?
Standard attachment to an eye-bolt going through a panel into reinforcement to spread the load. Where built-in attachment points weren't already available, obviously. In my Mini-Cooper the shoulder straps were anchored to the rear parcel shelf. My latest Lotus has the shoulder straps going to the standard rear seat lap belt points, and the straps live under the rear seat cushion for road/passenger use.
And as the one pictured only has I single action safety gate, I wouldn’t use it in a pink fit!