I received the final version today and it is good to go. It drops right into a stock M6 and works perfectly.
The overall length is less than 1mm longer than the stock MB20 so the switch activation pressure is not noticeably affected. The positive contact acorn nut now sits inside a recess in the end cap to help achieve this. It also looks cleaner this way.
The nylon thumb nuts are easy to grip and work perfectly.
The nuts added to the threaded rods keep the first cell from pushing through. When the second cell is inserted and centered, neither one touches the nuts anymore, so there should be little chance of the nuts wearing through the shrink wrap.
The only thing I would add is polarity indicators. A + and a - drawn on with a Sharpie would work but they rub off easily. Perhaps a very shallow hole, or a center punch mark, could be filled with a red and black marking pen. A minor detail, but it would be nice to not have to think about getting the cells in properly.
Good job Eric. I think that everybody who orders one of these will be pleased.