Hackers help: Sides for folder trolley?

I am very happy with my new trolley – have been looking for something durable to hold folders.  Just found out that I could screw in wheels at the bottom of the OMAR wire rack (it took me a while to find those wheels, but it was all worth it).

Does anyone have any idea as to what I could add at the sides to keep the folders from falling off? At the moment, I am using book ends, but they are falling apart.

2014-06-23 12.04.11

2014-06-21 23.27.04