What is the most reliable way to hold the battery module in place inside your flash?
That varies wildly from one flash to the next. I'm fond of drills and hack-saws. I just hack, buzz, drill, saw, or otherwise brutally remove the offending corner of my battery cover to accommodate the cable and roll with it. Be very careful that you don't chop off the the bit that holds the cover in place. Long, long ago Quantum used to include a sticky-backed velcro strip that sometimes worked.