Sumo posted a thread on Making Cables a while ago, (sorry TBone, I thought it was one of your tech pieces. It's on our list of stuff to add to the main site).
http://www.mankymonkeymotors.co.uk/forum/index.php?topic=5076.0Basically the same method you describe. On Reliants I usually buy a cable that has the correct fitting on the twist grip end, then cut the other end off as the SU carbs don't use a nipple, just a pinch bolt on the cable itself. Most old fashioned bike shops, or even bicycle shops will sell a universal cable kit that comes with a small nipple fixed to one end of the cable. You then choose from a selection of bigger nipples that fit over the small one, to suit your twist grip. Or use a screw on nipple, rather than a soldered on one. Either way would eliminate the stiff cable problem.