I use a C.P.A.P. mask for treatment of obstructive sleep apnoea. One of the clips for the straps that hold it on snapped so I threw together a replacement in Tinker CAD and printed it at my local library. The slot for the strap is 22mm by 5mm and the hook is 8mm in diameter, 2mm thick and 10mm wide. The STL file can be downloaded from here.

A photo of the 3D printed clip.