ahhhhhh. Okay. I see.
CO2 is heavier than air - if you're gonna age it for a while and don't wanna buy campden tablets, I'd say don do noffin.
A piece of sterilized aluminum foil, adequately smashed about the mouth of the bottle, would suffice in the short term. Or how bout some saran wrap and a rubber band? Inelegant, yes, but it will let your mead fart out however slowly with minimal chance of maiming you and still keeps the bugs out.
Who will try to ferment just about anything
"You guys just wanna get bombed and run around yelling yarr we're vikings!"