This is a corner clamp that uses Neodymium magnets to hold model parts together for gluing. It will hold two panels together for gluing, like small model buildings. The is a notch on the inside corner to help prevent glued parts from contacting the clamp. 

This is printed with PLA, which should be strong enough. Magnets are held in with cyanoacrylate glue.

This uses 12mm x 3mm Neodymium magnets for the clamping action. I used Part No. 07046 from Magnet Source [https://www.magnetsource.com/products/07046]. They have a pull rating of 2.08kg. There are many similar magnets on Amazon. Multiple file formats are included for modification of the clamp.

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

This is another clamp for smaller projects I designed to hold model parts together for gluing.

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

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

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

Here is another style of corner clamp, a square clamping jig that lets you a variety of clamps (C, spring, bar, etc.) to secure two parts together at 90°.

https://www.printables.com/model/840750-right-angle-corner-clamping-square