The support is the low-grade card cover of the sketchbook (see its overall technical notes), with overlaps from the mottled grey paste-paper outer cover evident around the top and bottom edges.
The technically similar Venice up to Trento sketchbook (Tate; Turner Bequest CCCXII), which Turner used immediately after this one, has a folding flap extending 57 mm from the fore-edge, folded twice to protect the page block and extend about a third of the way across the back cover. A similar flap was likely cut away freehand in this instance, leaving a slightly irregular, raw edge. Stubs of two green ribbon ties are threaded through slits at the centre and lower left. The Venice up to Trento book has a single corresponding ribbon loop on the back cover, but there is no sign of one here; either a loop was integral with the missing flap in this case, or the two ribbons were simply intended to be wrapped opposite ways around the book and tied to secure it.