I will echo dswtan and thank everyone for your insight to modifying a Lee WA adapter to use the Lee Filter Holder on the Canon 17mm TS, especially FM for starting this thread.

Couple things I learned in making mine:

I used the Lee 82mm WA adapter and like others, I cut out the inner ring as suggested. I then used a Dremmel tool to further cut away the remaining interior flange to make it flush with the inside diameter of the Lee Adapter. I wish I had taken photos of the before and after, but I didn't.

So after attaching the WA adapter to the donor Canon lens cap, which worked great, I put it onto the lens. Holding the lens back to a small light, with the rear lens cap attached, I found that while the WA adapter attached to the lens cap was "light tight". HOWEVER, I was bummed to find that the cap, attached to the lens IS NOT light tight. I could see two slivers of arch shaped light coming through the cap back side, that could contribute to reflections of light especially when using a Big Stopper during the middle of the day.

I tried looking for all sorts of solutions to block the light coming through the back side of the cap without interfering with the lens focus ring. After looking at several homemade solutions, I stopped in at Target, and bought the thickest diameter woman's hair bands (black) I could find. I slid one over the lens up to the back of the lens cap, viola, light blocked. And it stays in place as I rotate the focus ring.

Not saying any of you have this same issue with your lens cap, but maybe you have not noticed it before. If you do have the same issue, head to Target or the like, the solution is less than $1 away.

Hope this helps someone else! BTW, get the bands without metal on them, that could scratch your lens body.