(G) core plugs

hi
ive tracked my coolant leak down to one of the core plugs at the front of the engine above the turbo, i thought it was from the radiator from where it rubs against the fan housing.

my question is are these made from steel or aluminium from the factory as im a little suspect of the pinhole.

access is tight as ive tried sealing with jb weld might try again by removal of air filter housing, anyone know of a cost a garage would charge to replace? not a big job with the right tools.