A simple square window frame I made for the Enchanted Objects design challenge.
You should be able to scale the window easily if you are using OpenSCAD, simply change the parameters, the first is the size the second the frame thickness.
http://www.element14.com/community/community/design-challenges/enchanted-objects/blog/authors/Workshopshed

I needed a replacement window as the light from the house could not be seen. So I created this simple one with bevelled frames.