Somehow mine got built without this piece on one side. I thought it wouldn't matter much but the rack keeps coming out with kids banging on it every day. So, I modeled it in Tinkercad and printed it out and it works! No more shoe rack drawers falling out on the floor. It's not perfect but it gets the job done.
I printed it sliced in half and glued it together with Krazy Glue (CA). The screw is included as well. It's not terribly functional but if you're missing it like I was then it comes in handy. Installed with Gorilla Glue and a c-clamp and now it's rock-solid.
The author hasn't provided the model origin yet.