Matching Moment of Inertia in OpenRocket
I found this method in the OpenRocket Facebook group. The image and description are credited to Drew Nickel.
Pro-tip for more accurate dynamic stability analysis in OpenRocket: measure your rocket’s moment of inertia directly with a Bifilar pendulum, override each component’s mass to zero, and use two mass objects that together equal the weight of the rocket evenly spaced from the measured cg to match the measured longitudinal moment of inertia and center of mass of your rocket. Video tutorial on using the Bifilar pendulum: 
Updated YouTube link provided by Ezra Christensen: