Or you could just hire an electrician and buy new batteries so you don't have to worry about burning your house down. Lithium battery packs are perfectly safe to keep in your home when used and maintained properly.
It's not even so much the DIY aspect of this that's an issue, as much as it is people thinking it's OK to use old batteries. It's not. These are not deep cycle lead acid batteries. LiPo cells have a very definite cycle lifetime, beyond which their internal resistance grows to dangerous levels. Using them beyond that is just reckless and irresponsible.
Even new cells of this type have some risks, especially when the charge controller and wiring are all home-brew. Risk mitigation via physical barriers isn't at all a crazy thing to think about.
It's not even so much the DIY aspect of this that's an issue, as much as it is people thinking it's OK to use old batteries. It's not. These are not deep cycle lead acid batteries. LiPo cells have a very definite cycle lifetime, beyond which their internal resistance grows to dangerous levels. Using them beyond that is just reckless and irresponsible.