Tuning Passive Radiator

I am designing a box with an active element and a passive radiator. I am however unable to find a funktion that lets me att weight to the pasive radiator in order to tune it. Is there a solution?

Simulate your box with a port that's the same diameter as your PR.  It'll return a very long length.  Now calculate the volume of this huge port.  Next calculate the mass of the air in that port.  Subtract the PR moving mass from the air mass.  This is how much weight you should add.