Originally posted by PBandJ but I wonder if you can use a Cokin type system for filters on the 8-16?
No, won't work. The 8-16 doesn't even have any threads to attach filters (or adapter rings).
I've never used the 10-20, but I would expect that lens to vignette on the wide end with the Cokin P filter system. Even with the wide angle adapter, Cokin P filters usually begin vignetting around 13 or 14mm on APS-C lenses. You'll need a larger filter system for these ultra-wide lenses, like the Cokin Z-pro or the Lee filter system. That also means some fairly pricey, larger filters. You may want to reevaluate whether you need ND filters at ultra wide fovs; or start saving your money.