Hi, I am trying to find a 2mm thick filter for cevl.com http://www.cevl.com/mattebox.html matte box. This mattebox is not rigid, err its semi-rigid :) kinda cheap, but good quality cheap, I'll post my comments when I'm less of a newb. So:
it needs to be 4"x4" -2mm thick.
I particularly need a 'leave on' filter that is durable. I don't know what material is best. (Hitech has been recommended)

There's always Lee as well. Don't buy Lee in Canada as the distributor has an odd opinion of fair trade pricing.
http://www.leefilters.com/home.asp

What size lens do you have? A 3x3 filter will work up to .65 wide angle. 4x4 stuff is very pricey. You may be able to buy an adapter frame from Lee. It allows you to use P size on 4x4 2mm frames. Now P size is cheap like borcht.
http://www.bhphotovideo.com/bnh/controller/home?O=NavBar&A=search&Q=&ci=4034

Formatt make hitech
http://www.formatt.co.uk/home/default.asp
http://www.formatt.co.uk/hitech/filters.asp

Van Dieman also make resin but they're not really that popular.

Excellent advice, get the 3X3 adapter which will fit the matte box and take care of any rigidity issues, and the filters are cheaper!!
Is this what you mean?

The cardboard? adapter:

http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=87152&is=REG
or
http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=87155&is=REG

for the filter:
http://www.bhphotovideo.com/bnh/controller/home?O=NavBar&A=getItemDetail&Q=&sku=307137&is=REG&si=feat#goto_itemInfo

One more question:
Is 84mmx84mm = 3"x3" = P size?