We were gifted a beautiful Chanukiah for our wedding, and it has tapered cups. Last year, we bought a set of the medium plastic cups (Ohr Jelled E-Z Light, stock number 0613211450) that were filled with oil mixed with a bit of parafin so that it was solid. The set had cups that were tapered and fit beautifully into the Chanukiah. After we used them, we discarded them.
This year, we bought a similar set, but the plastic cups are straight-sided, so that they only fit into the tapered cups of the Chanukia at one point, and thus tend to tilt whenever the slightest vibration hits.
I can't even find a source for glass oil cups that are tapered instead of being straight-sided. Does anyone have any ideas for finding such cups, or for finding inserts that will fit into tapered cups?