I wish I could get a "trial sized" test kit that would do 5 or 10 tests, just to kick the tires. I hate buying an entire kit, using it a few times, only to realize I can't stand it. I might query the usual suspects (Salifert, RedSea, SeaChem) to see if they can accommodate such a request.
That being said, I picked up a pair of calipers today (needed them for another project anyway), and got a hair over 11mm ID on the tubes. I will locate the appropriate stoppers and post a note soon.
Edit: I'm thinking of these -
http://www.lcmlab.com/12mm_Thumb_Caps_Multi_Color_for_Glass_Test_Tube_p/210-0013w.htm
If there is no dissent, I will use these.