If you got time to source one the best overflow on the market is the LifeReef models. I have had good experience with the Eshopps brand overflows as well. I'm not a fan of the CPR overflows. They are designed to use the aqualifter pump to maintain prime which to me is a stopgap measure to fix a poorly designed overflow.
300gph should be plenty for a refugium. Are you going to gravity drain the refugium tank back to the sump?
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