I fear the neck alone precludes a kit version. You'd need far more accuracy to manufacture a tightly mating join than the factories the kits come from are currently capable of. If they used CNC machinery, then maybe, but as they don't, I really can't see it.

Also, the Stephen's Extended Cutaway® is a registered design, and is likely to still have a protected status requiring licensing (for commercial versions), for a few years yet.