I designed these for framing an LCD panel to turn it into a digital picture frame. I suppose they could be used for framing other things as well.
Adjust the parameters using customizer, export STLs for each of the four parts and print as many as you need. (I used four for the bottom edge, two of everything else.) Drill out the screw holes, pre-drill some holes into the frame and screw everything together 3/4-inch screws or similar. The holes in the design are for a 6mm threaded rod, which I use to hang the finished result on the wall.
Software, and additional build instructions can be found here: http://fredrik.hubbe.net/frame/