I hate leaving my weapon secured in the trunk, but sometimes it has to happen. To make double sure, get a box that can tether to the chassis/unibody somehow.
From;
https://oag.ca.gov/firearms/faqs
California Firearms Laws Summary pdf
See pages 15 and 16 regarding Criminal Storage.
These criminal storage offenses (first, second, or third degree) shall not apply whenever the
firearm is kept in a locked container or locked with a locking device that has rendered the
firearm inoperable. (Pen. Code, § 25105.)
Also;
Handguns
California Penal Code section 25400 does not prohibit a citizen of the United States over 18
years of age who is in lawful possession of a handgun, and who resides or is temporarily in
California, from transporting the handgun by motor vehicle provided it is unloaded and stored in
a locked container. (Pen. Code. §§ 25610 and 25615.)
The term “locked container” means a secure container which is fully enclosed and locked by a
padlock, key lock, combination lock, or similar locking device. This includes the trunk of a
motor vehicle, but does not include the utility or glove compartment. (Pen. Code, § 16850.)
Put all of that together and you should be covered, but I'm not an attorney.
But I did watch Law and Order last night