Fun project!
I have done a couple of plywood sumps.
This one is only 40" high, but uses WAY less wood.
http://www.bareefers.org/forum/threads/plywood-120g-sump-about-150.10015/
I have also built a few more complex things with epoxy, like a catamaran.
(fiberglass foam sandwich)
I have to admit, looking at the first page with crayon drawings and
the Lincoln Log type build, I thought it was an April fools joke also.
But once the extra skins went on the outside, no worries except if it leaks.
Glad to see you continued the build through the negativity.
I think you did exactly the right thing on sanding between coats on the inside.
Even if it was perhaps by accident.
Key for the inner layers of epoxy/fiberglass is to be water tight.
They do almost nothing structurally.
Key to that is to get rid of those annoying little bubbles that form. It is amazing how much water can seep through those.
IMO: Best way is a bit of sanding. It scrapes off the top of the bubble, allowing the next coat of epoxy to fill it.
Although some people swear by using flame. Eh..
I do have one suggestion though:
Consider puttiing a few strips of glass on the outside, below each opening.
Say a strip 8 inches high, all the way across, a few layers of good woven.
That joint between plywood and acrylic has high risk of leakage, and has (literally) a ton of load at the bottom.
Those extra layers on the outside would make it super-strong and if it leaks there, you are not relying on the wood.