Jason,
An Excellent effort!!
I am on my third Revision "C" of the EarthTainer design, so it will likely take you another spin or two, to get your system exactly the way you want it.
What adhesive did you use to cement in the overflow hole and level indicator?
Another approach you may want to consider to prevent the side "bulge" is to use one of those stiff 1" wide black stretch-straps (like a bungee cord) across the finished container. That should keep the sides from flexing out.
Keep us posted with your pictures if you switch to a more rigid outer shell, and what you then made your aeration bench from.
Ray