Building a super stealth water bottle battery!

Just read an article about this guy....
https://www.electricbike.com/mini-hub-motors/

How would one go about doing that? What parts would I need to build a water bottle battery like that? One that's actually inside a water bottle. One that you can take out and pop in if you park the bike somewhere. You could even build a second one if things are good and put it in your backpack so you get extended range and still good stealth!

Cheers and thanks for your help.

Most I could fit in a large waterbottle was a 7S 5800mAh 25C zippy compact. I have 2 waterbottle holders for a 14S setup. Making the batteries fit in the bottle wasnt an issue... its the bottle mounting mech that I didnt feel safe with.

I might revisit it in the future.

18650 cells fit in water bottle type batteries fairly well.