1. Have you seen the inner strap technique that Matt Newsome uses, which eliminates the buttonhole? There are some threads discussing it, and it is shown on his website here.

2. If you really want the traditional outer strap with buttonhole, I would investigate a technique called bound buttonhole. It is a classic tailoring technique that creates a beautifully finished and very durable opening. Here are some instructions, although I haven't reviewed them carefully, they should give you the general idea.