You can use a single or a double. The double just gives two leads and gives different tones and lights for each side. either one will work for a single mag.
You can also borrow mine if you want to fly up to 06C or wait until I next ask you to be a safety pilot for me Too bad I didn't know earlier. I flew right over you on my way back from Dayton on Tuesday.
For the o-ring, I just picked one out my HBF assortment box. You can measure it and go to ACE to get a good size. Pick about 1/16"-1/8" smaller in ID than the OD of the male threads on the dipstck.