Another great technique with the stock rain mat is to place it on an infinite plain, then tilt the plain up and face it towards the camera (track object). Then mess with the mat's frequency until it produces good close-looking results. Lastly, duplicate this plain several times and spread them farther and farther from the camera. I have found this to yield fair results, and the moral "higher ground" of keeping it all in Bryce feels damn good, too.