How can I make a tilt-shift effect with just basic blur?

Have tried messing about with both the "Blur" script and the "vinetting" script which has a blur. However the "blur" blurs the whole screen and I can't figure out how to limit the blur to just upper and lower part of the screen. I was thinking with a grayscale alpha or whatever, but don't know how to implement it in the script. The "Vignetting" effects blur everything in a circle around the center of the image and I can't figure out how to alter the shape in any way. I don't know if the shape of the "vignetting" effect is a scriptet vector or an acutal image, however I've tried to change som of the images in the Assets-folder under image effects without any luck. Anyone got an idea? I mean, it shoulden't be too hard, but my scripting skills are not good enough yet :( .. Any help is highly appreciated!