I designed these table connectors for my Prusa enclosure so I could connect a table to the bottom. Later I decided to build a storage shelf with the same connectors.
All the shelf takes is Ikea lack tables, wood screws, and the printed connectors. The price for this shelf was just over 30$.
For the best result I recommend using PETG for added strength. PLA will work but the connectors will crack if you're not careful.
To assemble line up the connector with to corners of the table. I recommend drilling a hole for screws first, but this isn't necessary. Next put a screw in the center hole then put in the side screws. Do this for each corner. After it is attached to the first table put the next one top and line it up with the connectors. Now screw each of the legs into the connectors. When you do this you will have to pull the legs taught and then screw them in to line them up. Repeat this process for however many tables you want on top of each other.
Edit: Added connector 2 which is the same as the first with the sharp edges removed.
Lack tables: https://www.ikea.com/us/en/catalog/products/20011408/