You must buy in super-bulk. My plugs cost more than a half-penny.
And fixing a set of earplugs costing a ha’penny with aluminum foil may mean the repair material cost more than the repaired object! :dubious:
TBH, I’d be worried (other than about zombie attack) that the foil would come off in the jack and stay there. Because it’ll never stay in a shape that’s helpful, so you’ll either have a wad of foil blocking the bottom of the jack (preventing full-depth insertion of the plug), or a loose wad of conductive material rattling around an electrically-powered device. That’ll be fun if it shorts the Li-Ion battery. :eek: