This is something I have been thinking about. The important thing is to have the emulsion of the negative in direct contact with the emulsion of the paper.
I was thinking about having a slightly curved base board (highest in the middle. Place the paper on it followed by the negative then place a heavy frame over it with two rubber strips on the edges which hold the negative down and pull it towards the paper.
In theory, perfect contact with the paper but no glass surfaces to keep clean.
I have not tried this... yet!
EDIT: Looks like I didn't read the first post properly!!
In answer to the OP's question - possibly not as flat as you think. Read Barry Thornton's book The Edge of Darkness.