(a) as "Trash" said, hold the filter directly in front of the lens ~ assuming your setup is on a tripod. Better still, get the larger-sized filter: 4" x 6" (easier to hold steady w/out intruding fingers) => no vignetting
(b) "rest" your normal-sized filter on the P-wide angle holder (w/out the adapter ring) => no vignetting at 17mm only if your holder-cum-filter is aligned properly. Trade off => as the filter is not sitting on the adapter ring, the filter-cum-holder may just drop off if you point your setup downwards.
I'm using both methods.
P.S. The expensive method => the Lee system (which I have not tried though because I have committed too much into Cokin/Singh-Ray combo) or the Cokin X-PRO system.