Discharge to a plenum with 300 to 500 FPM velocity at the cross section and provide with perforations 360 degrees around duct. See United Sheet Metal perforated duct. The pressure drop through the perforations must be much greater than the pressure drop through the assymetric inlet to make the distribution equal. However you may have to use (4) register type diffusers, instead of perforated duct to get longer throw. You can then balance each with manual volume damper.