On this thread: 15mm SK68 Nanjg 5*7135 V1.0 [final] I describe filling the gap between the pill and the thick battery tube with metal, with minimal tooling and material. It would be nice if someone with more stuff would make some rings of copper or soft aluminum and sell them.
The discussion ended up on that thread, because the ring is incompatible with using a 17 mm. driver, so building a good 15 mm. driver was by far the hardest part of the mod. There seems to be little difference between an SK-58 and an SK-68 around the bottom of the pill. I consider 1.8 A too low current to require cooling fins.