A simple, lightweight clip-on shelf divider for some 5mm glass shelves. Based on the design of the wire ones you can get from The Container Store, but I didn't want to try the metal on glass shelves. 

Created in OnShape; there's a larger version in there for those of you with larger print beds. If your printer isn't very well dialed in you'll probably have to snap the end of the hook off from the main body, but it's easy enough.