First, cigarette lighter plugs are a poor choice for any 12V application! Sadly, there really is no "industry standard" for a 12V appliance or extension cord.
If you want to make your own, I would start with marine, duplex cable. Not cheap, buy you can buy it by the foot on eBay in various gauges up to at least 12 gauge.
Second, find a source for Radio Controlled XT style connectors, preferably XT90. These are small, but very rugged. You can get bare connectors, but then you have to solder and heat shrink the wire to them. You can get them with pig tails so you can crimp them on. Use bare crimps and heat shrink.