Rear Seatbelts

I achieved this using a Kangol horizontal mount 4 generation "4Gen" belt
i have a back corner fix belt mount in close to the screen
i have a lower mount under the seat in the sill
and i have the usual lap belt mounts under the rear seat for the buck clasps
and i used these
Seat Belt Mounting Bracket Pair
as an offset to fit those reels in between panel and rear screen. basically converted a fix belt upper mount with no space into a slightly forward mount for the seat belt retractor

seats are fine for kids but a tall adult would bang head on the retractor in an accident
few adults comfortably fit in the back of an Australian charger and my kids are not all enough

if you have no rear shelf

Something like this bolted to a plate on the floor or under the seat would work
horizontal mount belts can be purchased in the US

Rock And Roll Bed 3 Point Automatic E Approved Seat Belt Kit Horizontal Mount | eBay

i.e you could mount it horizontal where you wanted and use a drop link like
this from an upper mount to smooth the belt flow over the top of the seat for the "customer"
SEAT BELT DROP LINK 150mm ADR APPROVED | eBay