According to this statute, just keeping the gun in a locked vehicle, even a convertible with the top up, should meet these requirements as long as you don't give a child the key. If someone breaks into your vehicle, child or otherwise, it would be a defense to prosecution under this statute because that entry would be illegal (see red above). I don't see any other legal requirement to keep the weapon in any sort of locked container or secured area inside the vehicle because just being in a locked vehicle should meet the requirements of section (a)(3) above.PC §46.13. MAKING A FIREARM ACCESSIBLE TO A CHILD.
(a) In this section:
(1) "Child" means a person younger than 17 years of age.
(2) "Readily dischargeable firearm" means a firearm that is
loaded with ammunition, whether or not a round is in the chamber.
(3) "Secure" means to take steps that a reasonable person would take to prevent the access to a readily dischargeable firearm by a child, including but not limited to placing a firearm in a locked container or temporarily rendering the firearm inoperable by a trigger lock or other means
(b) A person commits an offense if a child gains access to a readily dischargeable firearm and the person with criminal negligence:
(1) failed to secure the firearm; or
(2) left the firearm in a place to which the person knew or should have known the child would gain access.
(c) It is an affirmative defense to prosecution under this section that the child's access to the firearm:
(1) was supervised by a person older than 18 years of age and was for hunting, sporting, or other lawful purposes;
(2) consisted of lawful defense by the child of people or property;
(3) was gained by entering property in violation of this code; or
(4) occurred during a time when the actor was engaged in an ag-ricultural enterprise.
That said, I keep mine in a steel lock box secured under the seat when I have to leave it in the car. But, that's more to prevent it from being stolen in a smash and grab, not to meet any legal requirements.
Of course, usual disclaimers apply: IMHO, IANAL, etc...
Brian