Pete, these two Chinese makers are very helpful and worth having in your bookmarks. They offered to cut some to size for me when I had a project in mind and at a very reasonable cost.
Email them, or use the messaging system on Aliexpress and they will answer you questions.
http://www.alibaba.com/product-detai...124396578.html
http://www.alibaba.com/product-detai...600432925.html
There are some makers of quality filters on Aliexpress if you willing to search.
Hope it's helpful.
Trev