I just cut the last bit of CI out of my 4 inch stack, replacing it with PVC joined with Fernco sleeves. Now there is a bit of wobble that wasn't present when the CI was locked with riser clamps. I haven't yet connected branch lines to the stack and I expect that these will reduce the wobble. But until then is there some way of clamping/securing the PVC stack? I assume that the iron riser clamps are inappropriate.