The trinity kit is the easiest way. Other than that I wouldnt recomend just butt soldering them it would make it hard to get apart if you have to. A thin piece of copper works well as does battery braid. Al you do is lay the cells side by side like your making a ladder pack after the bar or braid is soldered on just fold the cells ends together. I made some for my starter box and just used double pieces of solder braid.