This is a small jig to help you clamp two parts at a 90-degree angle. You can position your part either on the inside or outside of this jig, and all inside corners have small reliefs to help you get a nice joint, even if your parts aren't perfect.
The legs of the jig are 3" and 1/4" thick, and 1" tall. You can scale this design to any size to fit your needs.
This jig was designed for these clamps, https://amzn.to/2XvftxG, which you can get on Amazon or from Harbor Freight.
I printed it in Hatchbox PLA, which you can get from Amazon. https://amzn.to/2RyN7NW
This design fits nicely on a Monoprice Mini 3D Printer. https://amzn.to/2Y23PHK
The file is designed to be printed with any layer height in the orientation shown in the renders. I've included the original CAD files, feel free to use or modify these any way you choose.