I don't know, but I've listened to the commentary too. He said that the guys who work props & sets are responsible for this tradition are nicknamed The Woodland Creatures. He also said the object is smaller than a bread box. He said it was larger than another object, but I don't remember what that object was.
I don't remember the exact name of the product, but there is an exercise device that looks a little bit like a school cafeteria chair with a third of it chopped off and no legs. It's got about 3 to 5 slots on each side where you can put your hands in different positions, and then you stick your butt in the middle. I think it's supposed to help you do sit-ups without straining your back or putting too much pressure on your tail bone.
Anyway, I've often seen this product used as the head rest on the back of shuttle seats in Star Trek shows and Babylon 5. I think in B5 it's used at the top of seats in Frazi class Narn fighters--and possibly in G'Quon class heavy cruisers.
I wonder if that's the prop. It's big enough that it wouldn't fit inside of a bread box, but if you melt it down it would be plenty small.