Got a small aluminum piece from a used Mac battery A1175

Grind both upper and lower side off the piece so it fit just against the right wall off MacBook

Drilled Ø 4 mm (better measure your own piece) and glued with cyanoacrylate the very far of the small metal tab.

Get a push-button small enough to fit below the mother board (beware of a thick piece (maybe transformer)) that can avoid placing your switch. My switch come from www.conrad.fr with ref n° 705325 (perfect fit at the chosen place)

Solder two thin wire ~ 4cm very close to the switch body, routing them so they don't touch the motherboard.