Im not sure on the Phase III post size, but the SL needs a 2" post. Something Ive done twice, that has worked well, is buying a 24" pc of 2" exhaust from an auto parts store and slipping it over the thinner pole. I then use stainless bolts and drill holes thru both pcs.
If you look at my SWM8 pics, you should see it.
If you look at my SWM8 pics, you should see it.