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Same issue as me then. I couldn’t get the freehub to free wheel with it on, but I was stressed as heck so might’ve made a mistake. My boss claimed that a colleague made the thing work with the rubber gasket installed, but I never confirmed. Your mileage may vary, I’d suggest installing it and see if its spins freely.
Would be worth taking a pic of the inside for folks here to double check/confirm.
It's very flexible, and its profile is round not flat...