I never had a spare battery that did not come with a cover or case. Did the OEM battery have a cover? If so maybe it will fit your aftermarket battery. A plastic bag will work. I would go for a more durable freezer bag instead of a flimsy sandwich bag. You may be able to cover the terminals with a piece of electrical tape. That would need to be replaced periodically as the sticky wore off. Anything to keep things that conducts electricity from contacting any two terminals at the same time. Some batteries are built where their terminals are almost like a plug receptacle making the chances of this happening very slim. Maybe that is why you did not get a cover with the battery.
If a Li-ion battery is discharged below a given threshold, it will be damaged. The damage can be in the form of decreased capacity, shorted life, or even lose the ability to be recharged. This will not happen in your camcorder. You camcorder will shut before it gets to this point. A paper clip or similar object can ruin a battery regardless of its charged state.