Typical deadener will do nothing on a box. You wanna deaden it? Build it right. Brace it well.
I'm undecided about rubber washers. The amp has to be tightened down very tight, and is therefore coupled to the box via the screw or bolt you mount it with, which basically negates benefits of rubber washers. And if it's loose enough for the rubber washers to do anything, then it's bouncing around so much that it's actually much worse. I know it sounds great, but I guess I'm just skeptical they're the saving grace everyone makes them out to be.