I bought some Lionel Part no. 6 109060010 Diaphragms for my Congressional cars after spending a fair amount of time trying to get something suitable. The part number referenced is for Lionel heavyweight cars.
I had to cut off the part that would normally mount to the door frame of the heavyweight car, because they would have been too close for making the turns (O72, O60, and O48) on my layout. I then used 3M 77 contact adhesive to glue them to the aluminum car.
I got these from S and W Parts Supply (http://www.sandwparts.com/) . I strongly recommend them- I simply called and asked, then he checked and told me he thought they would work, but I would have to modify them. He knows his stuff!!
