Hmm, its already in place so can't do a pic - its just stuff like a refill bottle for liquid hand soap or smaller standard shampoo bottle etc where you unscrew the cap and above the screw threads there's a thin cylinder neck.
You only need like a 1/2" or so. I found these types of products everyone has lots of around the house were in the right range. Just spend 10 mins take some calipers around and measure until you find approx the right one.
