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If noise is the main concern and you are only recirculating the filtered air, you might try the Noctura NF-A6x25 FLX fan. It is virtually silent.

Doubling or tripling the fan size would be very difficult with the present design. It would be easier to start over from scratch. Keep in mind that this application requires a fan that is efficient with high back pressure. Ordinary case fans will not work and I have not researched options for suitable larger fan.

In my case I have the fan in a significant enclosure that muffles most of the noise. In fact the motors and fans on m Prusa i3 MK2 printer completely drown out the HEPA Filter noise.