high concentration and long term storage actually don't go together--
freezer storage will probably precipitate, requiring warming before any measurement.
room temperature storage probably can get above 100mg/ml for some of the more soluble members of the family.
additionally, i believe due to the salt nature of the most commonly encountered 2c-x, that 50/50 water (vodka 100pf) is a far better solvent than absolute ethanol. deionized and distilled with no odor of course.