No idea about printing on cotton - just go hunt through the discount bin at the local fabric shop. There's usually a lot there.
Sure, cotton on the bottom should work, but it does add more weight, so I've only done it on top.
The rocker table is just an outer rectangle with a lot of cross pieces from side to side and they support the big flat piece of wood, which the plexi sits on. Some guys have it all adjustable. I just use shims to change the height and screw down through the shims to hold it in place. You can screw or bolt the frame together in many different ways.
Single to double concave?? Not sure, that sounds like it is a lot more complicated than the method I'm using. Maybe if you glued to the plexi in the middle and held it down in the middle somehow, then supported it further out with the long wood strip that gives the concave it might work. Sounds like too much trouble to me!
Has anyone here ever tried making 2 identical boards but one with a single concave and one with a double concave? I'd be interested to hear if it is more than just fashion.