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I would mix flies together in the alcohol and then push a ball of rolled tissue paper in behind them (gently) and then finish by pushing the stopper into the open end to seal it. You could also put some sticky tape around the vial too to make sure the stoper doesn't pop out in transit.

I think the standard industrial methylated spirit is about 75% alcohol and that is fine for months, maybe years. You just have to be careful because the alcohol evapourates if the stopper isn't air-tight :)