Ian
I use a sheet material called acetate which comes in A4 & A3 sizes and can be clear or coloured (or you can colour it yourself with spirit based inks)
Acetate is a card crafters material available from places like Hobbycraft
There are typically two thicknesses of craft acetate, Thin which is like tissue paper and Thick which is like a stiff paper and the one that I use .
I have loads of A4 thick clear acetate and if you get stuck I would be happy to post some to you.
Paul