This idea comes from the restaurant world. it's not new or original idea. it will hold paper by the pressure gravity has on marbles. which don't sound like a lot, but it's enough to hold 3mm worth of Polaroids with no problem. It allows you to just slide paper in and out with very little pressure or resistance. Or slide it down the rails. It's a way of keeping frequently used paper organized on a wall or shelf.

 As always, I have tried to provide enough different mounting configurations so hopefully one will work for you. one without any premade holes so you can put holes where you want if a suitable one does not work for you. You can just heat up a nail and punch it threw wherever best suits your needs. One version has a tail that could be glued under a shelf, the front lip could work as a backstop. 

You can super glue theme together. But if you want a little more surface to glue together, I have included a “combiner”. It's just a 1 cm sleeve that hold two pieces together. marbles will not roll pass a combiner/joiner, but it does not impede the function of the marbles. The end caps don't need glue; they are enough of a clip to stay put.  Also, I figured the top shouldn't go to waste, so I put a lip on top to keep a pencil or whatever from sliding off. It's a tiny little shelf, my gift to you. PLA works just fine. Mine are done in PLA Matte Black. I suggest something very heat resistant more than just Petg if using in an actual restaurant.