I use a gallon jug with a small neck. Obviously a tall glass like used for a yard of ale would be the best since a long column is preferable, but I don't get too concerned. The airflow from the diffuser creates enough movement of water to eventually contact every water molecule so I think the type of container becomes a moot point. I still think the smaller the diffuser pore size, the better contact with water molecules. You can get glass graduated cylinders from scientific places that are long and narrow if you're still concerned about having a column for ozonation.
Cheers,
Paul