There's not much you can do about Light Pollution except for moving but assuming that is just not the cards, I guess the only option to combat Light Pollution is a LPS filter. I see you are using an Astronomik LPS and as you have said, it doubles your exposure time. There is a great thread here on IIS about LP filters and I it might be worth having a look into the Hutech IDAS LPS filter for imaging as they are great filter for blocking out LP compared to other filters. The lens shade will also help but it wont make a huge difference.
A sling shot with a couple rocks will also work.
