Do you mean the upside down trapeze shape on the wheel?
I really wouldn't be using clay. It's heavy and really won't stick well regardless of what you try. You'd be better off shaping it out of something like sintra, or the cheaper alternative of craft foam.
I don't know how much the skates will really need to flex, as you don't exactly bend your feet in them, but look at something like Shoe Goo or E6000.