How to define a source in the shape of an octagon?

Hi all,

I am trying a few things out for my project and need to define a neutron source within two octagons (the inner one has d1, d2 and the outer one has D1,D2).

I was trying to build a source by specifying two cylinders with equivalent diameters preserving the volumes of octagons. Now, I need to perform the analysis for the case with explicitly defined octagons. Is there some way to create a source that fills the octagon-shaped region? Help would be greatly appreciated!

Thank you in advance!