This is a very simple mod and a completely reversible one.
Cutting two oval pieces, slightly larger than the black mesh with the logo on the outer side of the cups, of a magnetic sheet.
Since the mesh is made out of metal, the oval pieces can be attached to it magnetically. They have enough force to stay on, and enough flexibility to seal the cups just well enough to turn them into a semi-closed pair.
The mod makes them sound clearer and tighter, especially in the...