Yes Rubber Bands for the spider assembly's. There are ZERO off the shelf parts on here. I have a total of like $15.00 invested in this thing after glues, nuts, washers and threadall. Everything else was laying around (intertube, can, cups etc etc.)
The pole piece is magnetic steel. But there are 2 pieces of wood added to raise the height of the pole piece. I thought of it before. Then I saw that David at fixmyspeaker.com did it on a new school RE sub.