I bought an extra Cokin Z filter holder and turned it backwards, removing the three slots that are held in place by screws. Then I cut up a rubber PVC joint (made of rubber, not PVC) using a box cutter. Then filled the gap between the filter holder screws that stick out and the rubber joint with window/door weather seal. The rubber joint slides quite nicely onto the permanent lens shade on the D-FA 25mm and holds decently...though I almost lost it during high winds in Bryce Canyon a few years back. I used Velcro to secure the filter holder to the rubber piece, in case I ever decide to modify it or use it somewhere else.
There was another post on this forum about three years ago that showed someone else making a similar filter holder, only they used hot glue and plastic parts, instead of rubber. I sort of copied their design only I decided that I'd rather have something that is flexible and won't break if it gets knocked around in my camera bag...hence the rubber and Velcro. Hope this helps!