If you have the space for it, just put it outdoors 5' from anything flammable and you're good to go. This is not a hobby for folks in apartments unfortunately.

I've had one of my DIY ebike batteries short and fail spectacularly at near full charge and was able to push it with a broom out of the garage into the driveway before any damage was done. Now I have a bench with wheels that I can take into the driveway for initial testing.