It would seem to me you could also line at least the top of the inclosure with egg carton foam material as sold by McMasters. Could also add a layer of foam between the floor and the mill's base. I know the foam would be compressed in many areas of contact but will add some sound reduction.
A last thought is to look over some of the materials here.
www.b-quiet.com
They have a selection of materials made just for killing sound. Not cheap, but then moving isn't a low cost option it's self.
Looks to me like you have a tough job to kill the noise of a mill compared to most here trying to reduce the sound of a PC.
Good luck man.
