I went to the army surplus store and purchased a strong metal ammo can for $18, it mounts perfectly well on the rear rack of my bicycle. I have seen many people use soft battery bags or plastic toolboxes that shatter as the battery moves about. This ammo can should give you a better option in every way; it is cheap, durable and very effective. I did not fully intend to make an instructable so it's more like a documentary of an idiot trying to attach and modify a box to his bike and stuff his LifePO4 battery inside. Anyway, I hope it gives you some ideas to improve upon or implement into your own E-bike.
I would like to make this removable while making it secure, like if it could be clipped on or locked into place would be nice, should any of you have any ideas how to do this, or improve anything I did, let me know.
USE LOCK TIGHT ON THE BOLTS!!!!... Don't ask... :(