As in the bow compartment, the preferred material is PIR (Polyisocyanurate) insulation board, obtained from a builders' supplier.
I had purchased two full sheets of 25mm board for the seatbacks, thinking it would be more than enough. In practice I used all of both sheets.
As in the bow compartment I used a finely serrated kitchen knife to cut and trim the pieces of foam.
Here is the starboard seatback, stuffed with foam.
This is the view from above.
I find it reassuring to know that there is an entire 2.4 x 1.2m sheet of buoyancy in each side of the boat!