What I did:
-Create a LARGE plane in max and color it white(no textures)
-Grab a spotlight and make the cone of the spotlight go just above the plane (this should make a small angle and the triangular effect)
-you can tweak the hotspot/beam, and Falloff/Field effects of the spotlight
-render a top view, it should be overall black and a little white spotlight, then save it
-open up Photoshop and select the newly rendered image
-paste the image into the alpha channel in your texture, and stretch/enlarge to desired size.
-That's it.