On mine, the upper joint is corked (just like an ordinary wood clarinet) to accommodate the barrel. (http://www.hochstrasser.org/wiki/files/Rauber7.jpg)BobbyC said:
If yours is the other way round, cork the other part.
A usable solution on mine is to put one layer of clear adhesive tape along the inner circumference of the barrel. It's a bit tricky to do but will last about three to six months. A more permanent solution would be to paint the barrel socket with a thin layer of lacquer or epoxy. The quickest solution is to wind one or two rounds of cigarette paper around the mouthpiece cork before shoving it in.