Ben Egbert and I discussed doing a DIY filterholder utilizing the 17's front cap in April 2011. I am not handy so I had to wait until Ben actually did the work and produced a tutorial. (See below). Total cost for parts = $80 or so. You will not ba able to buy the Lee version (if real) for 4x this number. In the meantime, a bunch of folks popped on the internet with the same or similiar idea. Except for some vignetting in areas of the frame that appear when the shift is too strong, works like a charm.
Followed his work on another forum and he later said (I think?) that the thing fell of due to the glue not sticking hard enough. He also mentioned that he would not make another one since the front-cap with the standard-holder was better alternative. Smaller and less vinjeting.
If I'm not all wrong.