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.