If you need side supports for a shelf this is your model. To make it just print two times the same .stl (it is symmetric). There are two measures available (detailed in the .stl name) If you need any other fitting you can just easily modify the .step file that is also available.
Once done you can combine it with other models. In my case as you can see in the image I attached a mouse support and a keyboard support.
The author marked this model as their own original creation.