Interesting build. How do you plan to seal the gap between the window frame and barrel? Also, say its finished and holds water just fine will the bottom's round surface be able to withstand the pressure pushing down on it even if you do manage to support the entire bottom? Then how do you plan on working within the tank as it will be rather deep with what appears to be a very small working open area at the top?
I love the build, it's different and interesting those are just a couple concerns that came to mind as soon as I watched the video. Good luck, keep it updated!