I supply 18″ plastic rulers for my students. They are thin, cheap and fit well in our folders. They are, however, smooth and slippery…
Resulting in rulers sliding out of the folders throughout the day. They are identical so I just pile up found rulers to be recollected by students later. Its hard to find a tray or box for rulers to keep thing neat so I recently cut some lengths of PVC and capped on end. I have one for my lost and found, one for ink stained marker, and one for some nice rulers that we occasionally use.