I didn't really like the stupid SAE connector that you have to buy pre-made molded connector with whatever gauge wire they put on it. I got some Powerpole connectors to play with. They come with connectors (in different colors but I got BLACK and RED for negative and positive), then 15, 30, 45A contacts (that fit into the same connector) that I can mix and match to solder any size wire to it.

The 2 connectors can be separated or locked together in any configuration and they are keyed that the polarity is enforced.