I was posting this in another thread and thought it might be a good tip.
So you're looking for a place to put the big UWL-100 when not using it u/w? What I have done is I took the female end of a metal filter stack cap (the opposite threading of a normal lens cap) and drilled a hole in the center. Since the threading isn't deep enough on the lens cap to reach the threads of the UWL-100, I took a 67mm-72mm and threaded it on then a 72mm-67mm to get it back but give it some depth. I then threaded my mounting bolt for my ULCS tray through the hole of the lens cap and tightend everything down. I now have a place to screw my UWL-100 into when not in use.