Advantage Arms .22 Conversion units are generally considered the best around. Don't settle for any other.
Frankly, I have a problem with leaving guns in cars, particularly as you describe having to enter "bad" neighborhoods. All of us law abiding gun owners must do what we can to prevent lawfully owned guns from getting into the hands of criminals. Unless you have hands the size of catchers' mitts, perhaps a G26 carried on your person would be more effective and safer than a gun stored in the vehicle. Just my opinion.
If you plan to disregard my advice, then do this: mount a gun safe or sturdy lock box in the vehicle. I believe there are some made specifically for handguns. Second, by electronic and mechanical means, make it exceedingly difficult for someone to steal your vehicle. A firearms instructor in my division but in another state lost six gov't issue Remington 870s, six SIG 226s and several cases of ammo for both when the he stopped for breakfast on his way to the range to conduct firearms training. The gov't. vehicle was stolen from the diner parking lot.