I have the same unit I believe, version 1 (or 2 depending on where you look). Version 2 (or 3) does have a bottom vent and the bottom screws off as well.
I'm by no means an expert, and I don't recall where, but I did read that the bottom is designed to blow out in a battery venting event. It's also mentioned in
another thread here. I can neither confirm nor deny that it does, so take that for what it's worth. As far as protected
batteries go can you use one of the 2 Cents For Safety fuses?
Forgot to mention that AW 18650 IMR
batteries will work in this unit. I use mine as a charger in a pinch.