i tell you what i do have if your trying for extra rear seat/s with the extra jazz of being able to slide back n forth to suit...
I have a jazzy leather front passenger double (leighton van bentley stitch leather), the seat base was pro modded (fabrication specialist sorted for me) to make level and square so sits ocd parrallel in the rear... (front cab floor goes up towards front), it has the plate for inside the bottom so can store all sorts in there...and a pair of unwin rails and seat attachments and clasps so you can slide the seat back n forth plus put the ties in the unwin system should you want to bungee stuff down to the floor etc...
it was an idea i had for a van that then changed later.. so a lot of time effort and dosh went to waste. it was so it would go next to a pod in the back but be removable quick if needed, has its own belt built in but also another could be added later if needed to wall of van if you tried to squeeze two people on it.
to move it all on to a good home.... £250 ono... im after a smartpass 120 so if i've got money for that i'd be happy.