This is a simple and affordable Right Angle Corner Clamping Square that will give you a third hand when assembling any project that requires a 90° corner to be held in place. A big advantage is that you can print several of these at a reasonable cost.

Use any clamp handy (spring, bar, C, etc.) to secure the Corner Clamping Square while assembling your project. This clamp is 250mm on a side and 20mm in height. A higher infill will improve its useful life.

Start building! More:

A small vice that has multiple ways to secure objects, using spool pins or jaws. Versions with and without a base plate.

https://www.printables.com/model/1212482-small-universal-clamp-bench-or-portable-vise

I designed a triangular corner clamp for larger projects that can be seen here:

https://www.printables.com/model/466288-triangular-corner-clamp 

Another clamp for smaller projects I designed uses Neodymium magnets to hold model parts together for gluing:

https://www.printables.com/model/179135-magnetic-corner-clamp-for-model-construction

And one more of my corner clamps for smaller projects:

https://www.printables.com/model/230424-right-angle-90deg-corner-clamp