JNicks,
Anyone that can legally possess a handgun in Texas can carry it in a motor vehicle under their control in Texas. The Texas Penal Code only requires that the handgun not be carried in plain view. Under the law, a glove box, regardless of whether it is locked would be a proper place to store your handgun. Your portable lock box is not only proper under Texas law, but sounds like a solid instrument to protect your pistol from thieves.
A person who steals a firearm from a vehicle will likely be charged with Theft under Texas Penal Code § 31.03 or Robbery under Texas Penal Code § 29.02, potentially facing a felony depending on the individual circumstances of the crime. If your gun is stolen, you will not be held criminally responsible for crimes committed with the gun by the thief/robber.