[roundball]

I consider myself to be pretty safety conscious with firearms...and with my Flintlocks, I always use a thick leather hammer stall over the frizzen when hunting.
But for the vent, I make a concession to a modern item to ensure a fool proof safety when traveling with a loaded but unprimed flintlock in the vehicle.
I eliminate the possbility of any extraneous ignition source by firmly taping over the vent...a blue jay feather and all that sounds romantic/nostalgic but seems to precarious...at least for me...to consider it as a bone fide, trustworthy safety measure.

I bought a roll of Mil Spec OD color duct tape years ago, which uses a dry adhesive that leaves no residue.
At the start of hunting season I cut several 1/2" wide strips across the 2" width of the tape and have them available at my work bench, using a fresh one for each hunt.
I load the flintlock for a hunt in my garage before leaving, I press one of those strips of tape across the pan, up the side flat sealing the vent, and over the top flat to really anchor it in place.
When I get to where I'm ready to hunt, I peel it off, stick it to the stock on the off-lock side, and when finished hunting I brush out the pan and reapply the tape over the vent for the drive home.

Other tapes would work as well I'm sure, just mind the type of adhesive so you end up with a sticky pan area...for example, medical adhesive tape is usually a dry type of adhesive...the tape idea gives me 1000% more peace of mind than something like a feather.

That's just me...others mileage may vary