I got one of Bill's patterns. It makes a huge opening, almost to the body line, and not very attractive. After some experimenting, I think I will use something similar to the front openings. It isn't correct, but will be plenty large enough and decidedly more attractive.

I have a spare pair of front fenders that are pretty rusty. I am going to try and use the body lines above the front wheel openings on the rears. I don't know if it will work, but if it does, it may look factory.