I've seen it said many times that the stock HD600/650 cable isn't the greatest. I picked up one of the eBay cables second hand and chopped the end off and fitted an XLR socket. It certainly seemed to brighten up the headphone so its a worthwhile devour.
For my HE-500's i made my own using Canare star quad. For the heat shrink use whatever fits, i have a box of various sized lengths so i just used what fitted. By Cable pants i assume you mean the outer layer? Personally i dislike techflex for headphone cables, its fine for interconnects and power cables, but with headphones cables its too stiff and noisy when it rubs on things. The Canare and Mogami cable is so soft and flexible i generally leave it alone. If you really want to pretty it up use some paracord, its basically like a shoelace in that its soft and flexible too, and available in a range or army camo colours.