I am experimenting with making the absolutely simplest kind of accordion sketchbook - a long strip of paper, folded accordion style, and framed by a piece of thick cardstock at either end.
I cut a large sheet of Fabriano drawing paper into 16 cm wide bands, folded them, and stuck the sections together using a glue and narrow strips of strong thin paper. Next time I will use a spray mount instead of glue - the moisture made the paper buckle in the glued folds.
For the covers I cut pieces of thick, archival quality, acid free card stock slightly larger than the pages of the book - these extend 5 mm beyond the edge of the sketchbook pages.
It is possible to leaf through the pages, or to open the book completely into a long strip. Without a back the sketchbook is quite wobbly to sit with or hold in one's hand, and it works best with a flat surface to place it on while working.
I am thinking that a loose cover sleeve, with pockets for the stiff end-cards, might work when sketching out and about.