These were discovered by pSyk, and they work really well for being so cheap. They include a foam gasket in the lid and on the threaded side. The lid is spring-loaded and flips open once your depress the pushbutton. The ID at their narrowest point is .600"
He goes over using water bottle threaded necks glued on the OD of clear PVC in order to put these on a hopper magazine. I did the inverse and turned down some pipe couplers.
I turned the OD down to .995" then cut them in half.
I then filled the gap with 2-part epoxy (Plexus M310).
I ordered these for $5 each on eBay.