I too use the marine grade cables. #2AWG for under the hood batteries and #0 AWG for trunk mounted ones. #2 welding cable makes good conductors but the marine grade is STANNOUS coated for corrosion resistance. The coating looks shiny like silver but is actually good old tin. Make sure you run the negative cable to the ENGINE not the frame. Then run the flex strap from the fire wall to engine and a cable from the engine to frame.
I also use the red and black heat shrink with GLUE. This stuff totally seals the connections after you properly crimp the lugs on. Most electrical supply houses will crimp a connector or two for low $$.