To build the box, I used 8x4ft 3/4" plywood panels. When it comes to plywood, there are many choices, from very cheap (~$25) to better quality (~40) to really expensive hardwood (~$55). The main drawback of the cheap boards is that they come with lots of imperfections that might need considerable time to fix, and can even negatively impact their strength. This usually surfaces when you cut the boards and spot gaps.
For front, bottom, and sides I went for the middle-of-the-road, better quality plywood from a lumber place, where they also helped me with some of the long cuts. There is nothing that can't be cut just as well with a circular saw at home, but it is definitely easier to have this done with a large industrial saw, especially if they only charge like 50 cents a cut.
For the back, I was going to join two sheets, but then decided to use a different approach. I bought four very cheap, thinner boards (0.45") and doubled them up, so the join runs vertically on the back and horizontally in the front. Gluing the 60x60 board together was quite an interesting experience. I used a door as the base, since my garage floor isn't straight. Then I collected all clamps, buckets, concrete bags, and water bottles to press it all together.
It turned out alright, but like I mentioned, with cheaper plywood you get a lot of imperfections, and it took a couple of hours with wood filler and sander to get it all nice and smooth.
I bought 2" drywall screws at the local hardware store, and I used a couple of 18oz bottles of Gorilla glue to keep it all together. One very inexpensive way to increase the structural strength and assure the weight of the water does not pop the glue seams is to add triangular pieces of wood, ripped 45 degrees from 2x2's. This is where the table saw comes in handy... I glued these strips to all inside corners.
I opted for an integrated filter (and stuff) compartment. This makes applying the epoxy much harder, but I went for it anyway.
Main reasons: I don't want to use a canister or HOB or any such external filter. With the tank sitting in a garage that is not insulated, I think external filters would aggravate temperature swings, I would need more wattage in Winter, and the tank would heat up faster in Summer. Plus, with the tank design as it is, I wouldn't really know where to place it. Internal filters in the tank look ugly, and are messy to clean. Thanks to plywood I just added some length to the tank for 3 compartments (see schematic in second post): One for the pump (as well as heater, overflow to lower tank, and fertilizer reservoirs), a second center compartment for filter sponges, and a third "dry" compartment for some equipment, micro fertilizer, maybe maintenance supplies.
Braceless tanks make maintenance much more fun, and no light is reflected by dirty glass braces. For the top tank, I used a steel angle to add the necessary stiffness to the plywood.
The bottom tank is mainly planned as a fancy water reservoir at this point. It will get a shorter metal angle, combined with a threaded rod that goes from the front to the back panel to prevent any bowing.
One of my main concerns about possible weak points is that the top tank is mainly floating in air. When full of water, this is going to be a lot of weight supported by the sides panels. To prevent the bottom from bowing and ripping the front glass out of it's silicone bed, I added a 2x4 in front, 2x6 in the center, and a 2x2 in the back (where it connects to the back panel). Hope it will hold! I might add some more support from the outside.
Here is another shot adding the 45deg strips:
For easier access and handling, I am keeping both tanks and the back panel separate while applying epoxy. When done, I will screw the back panel to the tanks and apply epoxy to all the joins. The 60x60 double plywood back panel is very heavy, and when combined with the tanks I need another person to move it around.
At this stage I -cough- started to sand everything really smooth. Sanding sponges are my favorite tool, even though I used a finishing sander for some of the larger areas. Well, almost ready for some epoxy!