I was able to finish the front panel after work yesterday. Here's a picture, showing the front panel as well as the "access panel" which is leftover from the access panel cutout. Still thinking about how I'm going to hinge this....I'm going to try cannarella's idea and go from there.
Afterwards I cut the top inner-side pieces, of course mitered at 45° to align with the front panel. The top panel was cut afterwards, is perfectly square to align everything, and sits on top of the front/side panels. Below is the result, sitting on the "bottom section"of the cabinet. Keep in mind the entire top section will not be attached to the bottom section, so it can be easily moved off & away from the 90g for maintenance/aquascaping/coral placement/etc.
I used metal angle brackets inside at the miter joints for support, since there will be no additional plywood/glue/screw connections at the base of the top section. Even with these brackets, it still feels fragile. I'm thinking of ways to help beef this up, without losing the ability to remove the top section from the cabinet, while the tank is full and untouched.
Here you can see my bad miter joint...thankfully I'll be painting the cabinet, so I can fill this with wood putty, sand, and paint. Hopefully it will be invisible when complete. I'm worried, however, that when moving the top section, the stress on the joint will crack the wood putty/paint at this joint, and an ugly hairline crack will be visible all the way up my completed cabinet. If this is the case, then I will attach 1/2" corner trim over this joint.....but I'm hoping it won't be neccessary. Time will tell.
That's what I was able to accomplish in 2.5hrs....tonight I can hopefully work longer, since I don't have to work tomorrow. I plan on working all weekend on the cabinet, and I think it's reasonable to assume I can complete the build this weekend. Next week I'll start sanding, priming, sanding, priming, sanding, painting, and painting. I hate that part.
Take care!
Dustin