To fix vignetting when using cokin holder, buy a spare cokin filter holder (or a cheap copy off ebay) and saw the filter holder off after the 1st slot using a small hacksaw.
Ive just done this and now I can use my cokin circ pol and 1 more cokin filter (ND or ND grad) at 17 mm on my 5d without any vignetting. And I haven’t needed to remove my protective slim UV filter.
Great. no more trying to hold filters and keep my fingers out of frame.