I found a 14 awg DC Power Extension cable, 10 ft long, for $12 and change, at Amazon, and being a prime member, the shipping is free, so I ordered two one to use to make the cables that I need.
It is pure copper, and does have the 2.1mm x 5.5mm barrel ends, just like the 18 awg pigtails.
Also, this is even better than the pigtails, as the cables will have no splices in them. I can cut the full length that I need out of the new extension cable.
Of course I need a male and a female, and this will take care of that in fine fashion.