1. A customizable mask element for a user interface device ( 10 , 40 , 80 ), said user interface device comprising: a surface interface ( 11 , 48 , 88 ) adapted for contacting a user's face, wherein the customizable mask element ( 20 , 42 , 82 ) is either a cushion or a mask shell; and comprises: means for receiving and connecting a non-integrated customized element ( 15 , 58 , 98 ) in a manner wherein the customized element affects a shape of the surface interface ( 11 , 48 , 88 ) of the user interface device according to at least one body feature specific to the user, wherein said means for receiving and connecting the customized element is arranged outside of a breathing path of the mask element.