One of the simplest pieces of a bookbinder's kit is a folding stick. Used to fold signatures and press paper, a folding stick is basically just a rounded stick made out of any hard material. I've seen modern folding sticks made of bone, wood, and plastic. Some are shaped like pencils, others are more rounded. To stay within my theme of an 18th-century bookbinder, I did a little research on what was used at the time.
It looks like ivory was the preferred material for folding sticks:
"The leaves are first folded with a thin piece of ivory called a folding-stick, and laid over each other in the order of the signatures..."
"For any number of folds, however, a bone or ivory folder—a thin, smooth blade—was essential for rapid and accurate work."
For the correct shape of a period bone folder, the website of Jeff Peachy offers some measurements taken from period sources for French folding sticks. Basically the sticks were roughly 5 to 7 inches long, and sort of oval-shaped, tapering at the ends.
Using a sort of reverse description, this fits with other documentation that I found:
"Sepia, or the Scuttle-Fish, is found upon our Sea-Coasts plentiful enough: It is almost of the Form of a Spider, and so large, that the Bone taken out of its Body will sometimes measure six or seven Inches in Length.
This Bone which is the Part we use is flat, white and thin, much in the Shape of what the Stationers call a Folding-Stick." 
Looking up the shape of a cuttle fish bone does indeed show a long bone, roughly half a foot long, with rounded ends.
To make my own bone folding stick, I started with a dog bone purchased from the pet store. Using a hacksaw, I sawed off the flattest portion of the bone, then used the saw and some heavy-grit sandpaper to shape the piece into a rough rectangle. I then used sandpaper to flatten the top and bottom so that I could draw the shape of the folding stick. Again using the hacksaw, I rough cut a blank, then finished shaping with sandpaper, using finer and finer grits to get a nice smooth folder. My folder isn't quite as nice as a professionally-built one, but it feels nice in my hand, and should work for it's intended purpose.
One suggestion for anyone who may also attempt to built something out of bone: wear a mask. The bone dust is incredibly fine, and can get into your lungs easily.
 The Bookbinder in Eighteenth-century Williamsburg: An Account of His Life & Times, & of His Craft, (Williamsburg, VA: Colonial Williamsburg, 1959), 28.
 Temple H. Croker, Thomas Williams, and Samual Clarke, The complete dictionary of arts and sciences, Volume 1, (London: 1766)http://books.google.com/ (accessed June 11, 2012).
 The Bookbinder in Eighteenth-century Williamsburg, 29.
 R. Bradley, A Course of Lectures Upon The Materia Medica, Antient and Modern, (London: 1730)http://books.google.com/ (accessed June 8, 2012), 145.