Square filters for UW lenses

Cokin and Lee websites will be able to help you with the appropriate sizes you would need.
The vignetting is partly to do with the holder and how far it comes out.
However if you're a bit strapped for cash and you want a cheap fix in the meantime you can always cheat and use blu-tak to mount the filter directly over the lens. If it's wide enough to cover the front of the lens then you won't get vignetting :)

I use Lee filters and almost exclusively a polariser or an ND8.

Using the WA adapter ring I see little to no vignetting with the a900 and Zeiss 16-35mm..

Also they have a system where you can rotate two sets of filters separately so real good if you were every wanting to use a polariser and another filter like a graduated ND...

The kit is expensive but I would say that it is worth it... they also do over size lens caps so you don't need to keep removing the adapter rings every time you finish shooting.

Whatever square filters are used, they still have to be attached to the lens via its round filter thread. This is probably the weakest link and, dependent on lens design, can cause some vignetting. After that, you need to remember that the square filter is held some distance from the lens hence it needs to be just a little larger than a comparable round filter attached directly to the lens. For occasional use, you might consider a 100mm square filter just held in place manually during exposure.

Went to a shop the other day that had both Lee and Cokin P-series. They said the P-series would definitely cause vignetting. Unfortunately there is no dealer in my area that brings in the Cokin Z-series.

Also saw the cheap Tianya holder at another shop. Did not try that as it looked similar to Cokin design and same size as the P-series except that it is one piece molded.

I finally settled for the Lee foundation kit with wide adapter for my Tokina 11-16. There was some vignetting at 11mm with my round UV filter mounted. After removing the UV filter, there seemed to be no noticeable vignetting. Guess I'll have to use it w/o the UV.