It's definitely not from a Christmas tree bubble light.  They are flat
on the bottom to make the best contact with the bulb that heats them up.
I've never seen a level bulb that was not rounded on the ends.  Also the
color would make it hard to see the bubble.  And the bubble itself is
awfully big to be used as a level.  It looks like the ends were meant to
be broken off.  Perfume samples come in long thin tubes with similar
ends.  I would think the medicine vial suggestion sounds good.  Could
also fit in with a fort context.

Lee Cranmer