I don't want to keep the battery in my hot garage. At the moment it's charging in my office, right next to me. I wanted to get a Class D fire extinguisher, but they are ridiculously expensive for what you get.

In any case, if you know where I could get a fire proof container to store it in, that would be great, thanks.

It's 12 x 3.5 x 3.5 inches.

I have used an ammo box, something like this:
https://www.princessauto.com/en/deta...ox/A-p8791741e

They come in various shapes and sizes and you may find them in war surplus stores.

Echoing the above, ammo box. I store all of my R/C LiPo batteries in a box designed for belted 40mm grenades. You could fit probably six of those Shimano batteries in the same box with room left over.

Humpf

My first thought was also ammo boxes and rocket boxes too.

Another thing you might look at would be stonewear. Crock Pots, Bread Pans, Casseroles, etc.

I'm using an ammo box too since I don't have a fireplace or covered outdoor grill.

Any metal can, bucket, or cookwear would work. I use an old metal box that was originally for sending backup tapes offsite, similar to an ammo can. I store it inside my gas grill outside. Before I had the metal box I stored and charged my lipo batteries in the grill itself. It was always outside, protected from water, obviously safe with fire, and I had a plug nearby. Remember to take them out before starting the grill

Another safe charging aid.

Here is something I use to manage/limit the amount of time the charger is actually charging when doing it unattended (especially overnight).
Woods Countdown Timer