A prototype adaptation of the common book clip; intended for easier page turning on account of a hinge. Adjustable for large or small books.
Printing with two shells and 20% infill should do fine.
Print the parts.
Glue the two hinge pieces together if you desire (It can be nice but isn't necessary) .
Press the hinge barrel into the spine's hinge socket.
Push the clip head into the barrel's socket.
Refer to pictures if all else fails.