This is just a quick modification of http://www.thingiverse.com/thing:27184.

I liked the original funky design, but I needed to be able to screw it into two surfaces meeting at 90 degrees, so I added tabs and a stiffener along one edge.

The second variation has no stiffener and it can be flipped over and used on the outside corner of an object.

Scale to the desired size, print and enjoy!