Mmm well, one I don't know what the Yin Tang symbol is and can't seem to find images of it through googles and two, I don't know what you mean by sticking a Cetic fence inside it. lol. I can imagine it being hollowed out, but do you want it like a hole all the way through or not? ALso, I don't even know what a Cetic fence it to begin with. =P

If you do want to hollow it out though, you can always try booleans. Another easy easy way is to just use the split polygon tool to cut faces around the top portion, delete the inner faces, do an EditPolygons>FillHole command, then extrude the new face downward to make your crevice to make it hollow. (I think that's what you';re wanting to do... not sure lol)

Hmmm well, I hope you figure it out.