I have separate faucets for hot & cold in my tub, and want to convert to a single center mounted one. I have seen a plate you can get to cover the old holes, but my fiberglass tub has a lip about a half inch above the faucet handle trim. Do they make them that will work with this application, and where can I find them?

Can you post a picture? Would this work for you? (see image)

It works the best if you have copper or Pex water lines. With galvanized pipes you don't have too much room to insert tools through that plate opening and work inside the wall...