Hi 'Critter - you have really set the creaky wheels spinning in what passes for my memory!
As I don't throw things away I found the bits I cut off front and back - here:
Front buffer beam on the left in both pictures.
Fron these and the photos I deduce that I cut the front buffer beam off vertically, flush with the front face of the box-like structure, and the back one was cut vertically from the bit that will form the seat. Then the buffing face was cut off horizontally to leave the screw fixings, and this piece was split into 2 parts, one to fit in the base of the seat and the other to bridge the gap between the seat and the other side. Packing was needed on both sides to make the resulting structure rigid.
It was, you will appreciate, a 'cut it and see' approach to building - make it up as you go along, and what works is OK.
The easy bits - the end plates were from 2.5mm plasticard, 40mm x 28mm, and the bits along the sides were 1.5mm plasticard, 58.5mm x 7.5mm.
Hope this all makes sense. I look forward to watching your progress.