If you are not wanting to use glass you can get some clear film type material and use a vacuum frame that is say 12" wide by 6 feet long and cut a 1/4" channel about 1/4" Deep around the outside and drill it all the way through in one corner and attach a vacuum to it and then paint it flat black to take care of any extra reflected light. Then lay paper down then neg on top for the E to E contact kick on vacuum and roll out any bubbles and you have a quick and inexpensive contact frame. I used to have one that was 4'x8' just like that that I used for duplicating very large screen mylars.

Kevin