This is a very simple model for a flat connector/clip. The idea is to be able to use one or more of these on your model in case you want to quickly join flat parts without the need of glue or complex geometry. 

Simply download the STL file, add it as a part in the slicer, and then use the Mesh Boolean//Difference function where you want the joins to be. Afterwards simply print as many as you need for your project.

You can of course scale it to your need (or make them thicker for more stiffness).

You may need to account for clearance depending on your printer and the material, but so far I've been printing these as is and they usually click into place quite well as is.

I created these in order to be able to join parts for a model that was too big for the printer to do in a single plate. 

Basic print as example: