I might be able to expand a little on the method Ian G suggests, since this sounds like the method I use.
Ruby lith film is basically a sheet of mylar with a clingfilm-like layer of lith film. You score your mask on the film side and peel away the red film. The mylar sheet can then go between the neg and light source (film-side down). I have only found the film on *bay in the UK, but I'm sure you can get it elsewhere.
PS: Sorry to Ian G if this is not the method he suggests.