I know nothing of the solubility or molecular size of 7-OH. However, if it is similar to 7-OXO then try 15 mg/ml 6-oxo and 15 mg/ml 7-OH in a carrier similar to the forumula I posed in this thread.
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If you drop the 5% DMSO, substitute it with 5% DI water. You will find lots of other formulas on this board, but this one seems to help with the poor solubility of 6-oxo.