Note: an updated version of this listing is available, that offers both this original version, and one with the corner nipped off, as requested by some, to allow clamping even if there is glue or saw dust obscuring the corner. The new listing is available here: https://www.printables.com/model/60371-6-corner-clamp-right-angle-clamp-positioning-squar

This is a 6" corner brace (for clamping), aka right-angle clamp, aka positioning square.

These kinds of braces are great for woodworking, whenever you need to clamp two boards together at a right angle. Metal ones can be bought, although a teensy bit pricey. Plastic ones can be bought affordably, but they are only basic "L" shapes (having no diagonal strut for maintaining square). This one has the diagonal strut those plastic ones are missing. I added major ruler marks for inches and centimeters, and minor ruler marks for 1/2", 1/4" 1/8" and millimeters.

No supports are necessary, assuming your 3D printer can do even a small distance of bridging. Speaking of which, notice that the STL file is positioned 45 degrees from how you'd assume. That's to help with the bridging. Otherwise, your slicer would likely be trying to bridge the entire 6.8" span of that aforementioned diagonal strut in single bounds. By having the thing turned 45 degrees, it should only need to bridge about 0.38" at maximum.

PS: Obviously if you scale this up or down, the ruler measurements will no longer be valid, however you'd have the new size you desire. If there's a demand for other sizes I could produce those with ruler measurements still intact.

This was created from scratch by yours truly, using SketchUp Pro.