I have found a one piece lid to be WAY easier than two pieces, BUT, you obviously don't have a piece of wood thick enough, so ya gotta do what ya gotta do.
The piece of wood you have is plenty big for a box lid if you're careful.
Usually, I make mine about an inch and a half wide at the rear end, but that's with a great big early gun. You should be fine with what you have.
Just draw it out on the side of the stock with a pencil and see what it looks like!