AC's have all kinds of tricks to rig up shoulderpads for arri base plates. I have never actually used a premanufatured one. i used to use neoprene gauntlets that i got when I worked on a crab boat in highschool. Just shoved them under the base when we go handheld. Element Technica is supposed to have a cool one in the works. They make the RED arri base plates, and other fine aks.
i've been thinking of making a delrin sliding base plate with a shoulder pad underit. So you can pull the camera off the Arri plate and slide in the shoulder pad. We'll see how good element technica's is.