a fastener for use in air massage systems used with a bathtub that includes a shell having at least two layers with at least one hole being defined through the shell . the fastener , for instance in the form of a rivet , comprises a hollow body adapted to extend through the hole in the shell and a securing mechanism provided for anchoring the fastener in the hole and for retaining together the layers of the shell . the securing mechanism can comprise enlarged distal and proximal elements provided at opposed ends of the hollow body and adapted to be disposed against inner visible and outer hidden surfaces of the shell . the proximal element includes a decorative head adapted to at least partly overlie the visible surface of the shell .