Rear seat bench
I created the 3 person rear seats for my daughters and their friends, now that they are 5 years older it is a 2 seater with a little space in the middle for them, but not for three people anymore.
All you do is add a spacer aluminum piece over the gap between seats, than make one seat wider, hole in the wider seat for extra seat belt. I used the same brackets and installed two seat belts to the two inner brackets making them slight wider before coming up the hole in the wider seat and in between the regular seat and wider seat. Because it was for children I wasn't concerned with strength limitations. Now there are only one per adapter (or one seat belt connector per seat) to allow for the two larger people in the rear.