Lightest Refill Tank?

Since your Benji tank is made by Ninja it should not be metric thread. I would look into finding one of the Ninja 45ci "peanut" tanks. I think this would be your lightest option. Just get tank without a reg and put yours in it.

Ha - That I didn't know! I had a Ninja loaded up in my cart, but started reading into various issues around quality / safety, and some unverifiable stuff around some of them not having DOT markings. 



Ninja makes a SL (super light) small paintball tank in 68 cu in that would work fine for your intent. You can find used ones on eBay fairly cheap. Threads are 5/8"-18, same as your Benji tank.

Thanks, the other thing that I need to get my hands on is a male DIN fill adapter. Learned that the hard way today, the closest shop to me that has one is 31 miles away :/