Suitable lap-belt such as the Securon 210C (other sources are available ...)
Central mountings behind the rear seat, under the spare wheel cover.
Slot in the rear edge of the spare wheel cover
Boss on the wheel arch beside the seat
Seat belt end on the bolt first, then the wavy washer, then the stepped spacer. The two previous components fit over the narrower part of the spacer. Bolts seem to be 7/16" UNF/20. They should be short enough to clamp the spacer tight to the body, while still engaging as many threads as possible i.e. if they have to be shortened don't cut them too short.
Quick and neat installation. Using the original boss for the buckle half the belt would have to be adjusted to put the buckle beside the opposite hip of the passenger, otherwise it will be across their abdomen which is incorrect.