I know California has changed since I lived there (not too long ago); when I went through the Gun Safety and Hunter Safety classes they told us that "Without a Conceal Carry Permit, all firearms had to be transported in plain sight and not loaded" - we were told that if it was in a case that made it "Consealed" even if the case was locked. So that is how we carried ML too and from Events and the Range.We have the same law in California. I ignore it. I put guns under the floor carpet in the back of the vehicle. Around here, if you are a "good guy" not a gang banger or meth head, the cops will not confront you on this. Especially if you are obviously going to the range. First they need a reason to pull you over, so don's speed or drive crazy.
Maybe ask an officer or two at Starbucks or Dunkin Doughnuts and get their opinion. I doubt they will throw up any road blocks for you flintlock. Which, btw is not considered a firearm under Federal rules.
A trigger lock is sufficient here. You could also put the lock in your shooting box.
I was once tasked with storing Everyone's rifle overnight for an event; drive from Long Beach to Pasadena with 15 rifles all piled in the back....and prayed I didn't get pulled over for anything (I had a couple pounds of powder, No lead, several swords, daggers, and dressed like a Pirate)