Here is an easy and inexpensive way as requested:
http://www.oldpinballs.com/store.html
Item Code: PFRD (about 3/4 down on the page).
These are thin clear vinyl disces of different diameters you can build up to minimize the cupping. It does work to stop the ball from getting stuck. You will see a slight ghost outline of each layer but that actually looks cool!
This is also undo-able if the next owner doesn't want it.
Mike V.