Why would you want to use foil? All it does is cover (loosely) the opening. For about $3 you can buy a drilled rubber stopper of the correct size & a plastic airlock to fit it; add a wee bit of tap water & you've got a safe, reliable airlock that keeps nasties out & allows CO2 to escape. Maybe the foil will work well enough for you, personally I wouldn't risk it; I put too much time, effort & money into my wine/mead/cider to risk it. Good luck, GF.