This is what we have in our 1977 TE Cortine. In this particular vehicle, the handbrake sits directly in line wuth the auto selector. What we did was we got a piece of steel pipe that was just large enough in diamater to fit over the handbrake grip without being loose. We cut the pipe a bit longer that the grip and drillef a hole through it through which you put a padlock. Now you get a steel rod and weld a U-shaped piece of rod which is the right shape and size to fit around the auto selector shaft withoug being loose. you weld the other end of the rod to the pipe. Basically how it works is you put the selector in park and pull the handbrake on. Slip the pipe over the handbrake and slide it forward so that the 'U' on the end of the shaft pushes against the selector shaft. If you have drilled the hole in the right spot the padlock will prevent the assembly from being slid back over the handbrake grip. Basically what this does is locks the handbrake and gear selector against each other. Although the whole system relies on a padlock, you can buy locks that, when shut, leave very little cutable material exposed....plus if the thief has never seen one before he might be a little confused for a bit....I know I was when I first saw dad use it all those years ago
I know that Commodores don't have the grip in line with the selector but I reckon that with a bit of careful engineering you could make this work..
Oh and my apologies for the long post...