I was looking at some fresnels screens from a UK supplier (not specifically photographic) and they were 1.5mm thick so that makes sense.
My APUG portfolio
1.5mm is about what a fresnel is typically, so that sounds right to me as well. You can just shim it until you find the right fresnel.
Photography (not as up to date as the flickr site)--http://www.davidagoldfarb.com/photo
Academic (Slavic and Comparative Literature)--http://www.davidagoldfarb.com