Ok, here the progress with the tank itself.
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. When I first started out I was using just regular West System 105/206 (one coat to saturate the wood and a second coat for excess glue). As the build progressed I started to thicken the second coat with Cabosil and I highly recommend doing this. The thickened epoxy doesn't squeeze out as much and lets you load the joint with more glue.
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. I used clamps to hold the sides together while driving in the screws. 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 back and left side are just 3". The front and right side are 4" wide and I used a coping saw to make cutouts for future fan access.
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.
Here's the bottom installed
The bottom is glued to the sides and to the lower bracing with thickened epoxy. It is also screwed into the bottom bracing with screws every 2" and also screwed to the sides with pocket holes (staggered relative to the pocket holes in the bottom bracing).
You might recall I mentioned near the start how I discovered that "3/4" ply isn't actually 3/4" (more like 11/16"). Because the sides are joined with miters and my bottom piece is inset into the sides, this meant that the bottom was 1/8" too small. To deal with this I cut some 1/8" slivers from some scrap poplar and epoxied them into the gap. The pocket holes are actually driven through some of these poplar shims. I think this has addressed the problem quite well.
I filled all remaining screw holes and gaps in the bottom with wood putty, sanded and them painted with 3 coats of Drylok. Here's how it looks:
For those of you know don't know, Drylok isn't smooth like paint. It's filled with bits of sand. This means the resulting finish is quite rough and can't really be sanded for a smooth finish. Here's a closeup: