I don’t know about the original size, I tested a smaller driver and used the bare stripped original as a contact plate: Mod: Small Sun ZY-T08 MT-G2 (without cells in series conversion)
I didn’t like that driver though, so I’ve made a driver and a tail switch plate in OSH Park especially for this light, making series conversion easy. I did the tail switch design so that the bigger Omten switch fits perfectly over the spring solder points. I’ll be updating that thread when I eventually get around to putting it together.