This design is thanks to @snookben147

This is a simple, but strong plastic connector system for building custom and modular furniture, from rectangular wooden rods. The square version of this model, using a 17mmx17mm wooden profile. The same item can be used to build a table, shelf, or even architectural structures.

To build a cabinet, you will need 17mmx17mm (18x18) wooden sticks. You will need four pieces of lumber and four connectors to complete a frame. To fill this frame, you’ll need to cut a wooden surface out of plywood. For a more aesthetic result, cut the edges and corners at 45 degrees, and the board will fit perfectly into the frame hole.

You need 16 wood screws to secure a frame and 4 smaller ones to secure the plywood. Repeat for the required amount of surfaces. Once done, slide the furniture legs to the desired height and screw in place with the remaining holes of the connector.