You really don't need an airtight seal on your fermenter. The co2 coming out of there would protect your beer. In fact many folks with arthitis and other issues don't snap the lid down on their buckets anyway, and may folks just put tinfoil, plastic wrap, metal cookie sheets or even plexiglass sheets on top of the bucket instead. It's really not crucial to be tight. The bad stuff are not ninja acrobats, they really can't get into stuff. The co2 coming out will prevent anything getting in.
In fact you don't want a "pressurized" bucket, if you have one, then at some point you will end up with a ceiling full of beer. I've had that happen when the vent (airlock) gets blocked by a freak hop cone, and it wasn't pretty.