The supplied STL/step is made for:

• A square socket with sides of 78mm and a corner radius of 5mm. 
• The shelf length is 90mm, depth is 25mm, thickness is 4mm.

The ring slides around the outside of the socket with a friction fit, no glue, tape or screws needed.

The model is fully parametric and can be easily adapted by changing the parameters in Onshape (account is needed = free). The parameters/detailed dimensions are shown in the pictures. 

(to export the step/stl file of the adapted model after changing the parameters in Onshape, just right click on Part1 and select "Export")

A step file is included to make small modification easy.

The Onshape link can be found by downloading the text file 

Printing: Using PLA or PETG is sufficient. A mouse ear at the end of the shelf or a brim could be added if the build plate adhesion is not sufficient. No supports are needed.