Roll over cost will kill you........ There's no efficient long term investment vehicle in commodity markets.
If I were to do a swing trade, I would wait for freezing winter, Russia to turn off NG supply to Europe, some disaster that affects NG production. Right now, the supply-demand is far too stable.
I try not to chase price, price has deviated greatly from the mean, pricing in next winter could start now, has been known in past.