Ok, I've been really busy with other stuff so progress has been very slow but here's a long overdue update.
As I mentioned earlier, I do not recommend using a mitered corner design like this. The joint lacks structural strength and it's very hard to get everything to line up. That being said, I decided to make the best of what I had and modified the edges to create sort of a haunched miter. This would offer a better supported joint with a much larger gluing surface. Here's the plan for how I hoped the edges would fit together.
I began by epoxying and screwing some 3/4" strips of plywood to the edges of all the pieces.
Next I applied a coating of epoxy to all the joints (first a regular coat to saturate the wood, followed by a second layer of epoxy thickened with Cabosil) and screwed them together with 1.25" and 2" wood screws as shown in the diagram above.
Here's a closeup of the joints to show how they fit together. There's a screw every 2" but they're spaced in an alternating pattern. As expected, the mitered part of the joints didn't fit together quite as perfectly as I'd hoped leaving a bit of a gap on the back edges where I couldn't produce much clamping pressure. I solved this by injecting epoxy resin into all the gaps to produce a solid, epoxy-filled joint.
For bracing around the top edge I installed some strips of 3/4" plywood. The front and right side are 4" wide with cutouts for future fan installation. The back and left side are 3". I attached the strips with Titebond III and pocket hole screws. This is really strong - I did a set of dips supporting myself just on the bracing and it didn't budge (though I admittedly don't weight very much)! Eventually I'm going to add an additional 3" center brace running from front to back.
Here's the tank flipped over and the bottom bracing installed. Here I used 1X3 poplar strips, epoxied and pocket hole screwed like the top. The difference here is that the strips were attached 3/4" away from the edge, so that once the 3/4" plywood bottom panel is installed it will be flush with the bottom edge of the sides. You can see the bottom panel leaning against the wall in the background, pocket holes drilled and ready to be installed.
That's it for now. I'll add some more updates when I get them off my camera.