It sounds like you already tried an adaptor without success. I'd check with your supplier to see if one is made that will work for you.
Another solution would be to build a flat spot into your ceiling to hang the fan from. It would be a lot of extra work and I'm not sure how it would look.
You'd have to have a very firm connection to the rafters. You could cover with drywall or wood trim whatever fits your room.
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