I know there's a couple people on here who ferment in the smaller (narrower) ones and if you look in the latest byo in the JOHN PALMER article you will see the Aussies use one similar to the narrow ones..
This 7 gallon is intriguing, but I will add some caution about the ability to clear all the krauzen from the gaps. I often use a 3 gallon Better bottle, which is square as well, but it is clear...
Krauzen does get caught in the squarish shoulders of it, and unfortunately my jet carboy/washer cleaner even has a difficult time hitting it...BUT oxyclean, water and vigourous shaking and rinsing does free the junk...
But I can see it....
I'm not gonna dissuade you from using it...I love innovation, and you are not the first on here to ferment in them, and now you got me wondering about larger sizes....
What I would do though is get an illuminated dental mirror...
AND a flexible LED light
and use them both after you clean in out to make SURE you get every piece of biomatter out of it.....Even sticking the flex light into that back drain and use the mirror in the bigger mouth.
If you are a sloppy/lazy brewer I wouldn't recommend using one of those aqutainers.....but if you have a bit of anality about it and use a flex light and mirror, and methodically look, and rinse and look again, you should be fine...
You may even wanna make a bendy cleaning tool, like cut up a sponge (not a rough scrubby but a natural sponge) and mount it on something that you can stick in to use to clean out any corners...
Have fun and keep us posted at cleanup....