protective film for exposing carbon prints?

I'm setting myself up for carbon printing and am not sure what protective film I should use between negative and carbon tissue during exposure? Suggestions....

Thanks.

I use clear acetate sheets from local art store.

Thanks Andrew. I'm almost completely set up now. Did other projects over the last year or so but I set aside time in July to give Carbon printing a go. It's a very "busy" process compared to regular silver gelatin printing. My fingers are crossed...