Ditto on the Higley. I've given up on the Top Flite tool. 9/10 times I just use a fresh single edge razor blade and use my fingers as a guide (Best done before 1 am and never after the 3rd cup of coffee!).

For covering over covering trim, go to your local Art Supply store and buy a sheet of mylar. Trim it to a workable size and put it between the covering you want to cut and the covering you want to protect. Cut an additional strip to use as a cutting guide and lay over the piece you want to cut. Mylar is great for following the contours of wings.