You shoudl be able to fit three SubCs, rather than 4/5 subC with some modding:
1) If you have access to machining, bore out the tailcap
OR
2.a) Buy a Fivemega stainless steel tailcap. (as a bonus, allows easier tailstanding)
2.b) Use a dremel tool to remove as much material from the bottom of the switch assembly as possible.
2.c) If necessary, use the dremel to remove material from the TOP of the switch assembly as well, so that the whole assembly sits higher up when pressed against the retainer ring.
By doing this, you will double your runtime compared to the 4/5 subC.