Making a Cat Window Perch?

I'd like to hear ideas about how to make an inexpensive, yet durable cat window perch. Or if anyone has ever made one, how did you do it? My 7lb. calico loves to sit with me at the window when I am at my computer, but during the winter, the window is closed and the ledge is not big enough for her to sit, lay, or stretch out. I would love to here a Do-It-Yourself way to build one.

By Vanessa