Personally I'm a fan of your solution similar to something I would have done.The car is still new to me so I'm still playing with it ... I prefer sport mode so if I forget to switch prior to pulling away from my home then that's when I'm most likely to accidentally depress the Power button.
You guys can all say what you want, but the location of that button is a poor design. That would never pass muster in any aircraft cockpit. Sure, the Volt is not a fighter aircraft, but if inadvertently switching the car off leads to an accident the consequences can be the same.
This topic pops up often enough so obviously I'm not the only Volt owner with this concern. I was only trying to offer another inexpensive solution.
GM seems to have learned their lesson about the power button placement, as in Gen 2 it's now behind the right side of the steering wheel and nowhere near any other pressable buttons. The Mode button is now aft of the shifter.