Dude. Don't sweat the seal so much! You're really over-engineering this, because you're overthinking it.
Just cut a square of plexiglass that'll cover the entire mouth of your bucket. Clean it and sanitize it with the same stuff you clean/sanitize your fermenter. Knock out, pitch yeast, place the sanitized sheet of clear plastic over the bucket.
Done.
You don't need an airlock. The devolution of carbon dioxide gas during the ferment will displace any air in the bucket and excess CO2 will get out just fine. Nothing will get in, no solid matter, no microbes. Really, the sheet is there to prevent dust and stuff from falling in; the krauesen and layer of CO2 gas will protect your beer from airborne microbes.
KISS. Really. Honest.
Cheers,
Bob