I have been searching for a way to use my existing Lee holder (Lee Foundation kit) + Lee filters with the Canon 17mm f4L TS-E. I wanted to be able to use the same holder with all my other lenses.
The idea was to use a spare lens cap glued to a 82mm WA Lee adapter.
We found an easy way to accomplish this and the entire process has been described on this thread:
http://www.fredmiranda.com/forum/topic/1081875