Speaking from personal experience, I strongly recommend against leaving the portable charger in the trunk.
You may be OK if you do it occasionally with proper precautions, but it's not a good long-term solution. There's nothing built into the car to accommodate this.
When I got my 2016 eGolf, I was worried about the Level 1 charger getting stolen so I did exactly what you propose: I left the Level 1 charger in the trunk, ran an extension cord into the trunk, and closed the rear hatch over the wires.
I started with a 14-gauge extension cord. It was quickly obvious by the plug getting hot that the cord was not thick enough. I switched to a 12-gauge cord and was fine after that.
The first time it rained hard, the seal on the rear hatch leaked and I had a couple of inches of water in my rear seat wells. I've found that having something even as thin as a leaf caught in the rear hatch seals is enough to let water in.
The portable charger would get hot to the touch when in the trunk, and over time the plug began to melt. After that I shelled out $300 for a new portable charger and never left it in the trunk again.
As others have said, on the 2015 SEL the plug locks to the car so the charger can't actually be stolen. However it's certainly possible that you can get some expensive damage by someone attempting to steal it.
So in your situation, what you'll want to do is:
- use a 12-gauge extension cord or thicker. Remember in this case that lower numbers are better. A 10-gauge cord is fine, a 14-gauge cord is a hazard.
- Leave the Level 1 charger outside of the car. If you have the same black Delphi portable charger that I had with my 2016, know that it runs hot under normal operation and dangerously hot if left in the trunk. My 2019 came with a white charger that runs cool so it doesn't have the same problem, but I still leave it outside the car.
- I now hide the charger out of sight behind the wheel. So far after several years I've been fine. Because the 2015 SEL plug locks to the car, you may just want some obvious visual way to show potential thieves that it's locked. As f1geek suggests, that could be a cable lock wrapped around your wheel spoke, or maybe a magnetic weather guard that hides the plug from view, or maybe there's a piece of wood nearby where you could screw a metal loop and lock your cable to that.