Did you buy this brand new or used? If new, Fluval had good customer support helping my mom with an issue on a 304 years ago (something to do with the handle/clamp IIRC - not a gasket issue, but an issue with their product no less).
If you bought it used, well, you bought it used. Contact the seller or figure it out yourself. Maybe taking the filter into Highland tropical they can help better, but, and not suggesting you would, I wouldn't be rude about them selling you the gasket which is also too large. Something doesn't add up.