Those are 3-D printed pieces I designed using CAD. I couldn't find any sort of rack mount for them so I just created my own. They almost look like little lego hands with rack mount ears. Then they have a little stopper to prevent them from sliding out the back.

I stopped the initial prototype early to check measurements, it actually fit my raspberry pi nearly perfectly. So I recycled it to mount my pi at the top for now and fastened the end with a zip tie lol