I am in the process of building a very similar build (that i ended up modifying from the original, which looked almost identical to that)
I won't say that it is at all necessary, but I would recommend that you consider building your frame of 2x4's and a plywood "shell"
My build is 3'x3'x8' (overall frame size) and made of 1/2" plywood AND 2x4 shell and it is still NOT STURDY (it wobbles) so i am adding a few angle braces just incase.
I'm not sure why you are using the aluminum? If you want the front to be completely glass/open you can use wood dowels for the same effective without ruining the consistency of the outside "look"
Are you planning to put epoxy on the wood and thats it? pour your water straight over it ? I know theoretically that should be fine, but I always over-design failsafes.....just in case it fails in the future...
Here is a pic of my current build for reference: https://plus.google.com/photos/11015...jAE&banner=pwa
I am using wood "mounts" attached to my back wall to elevate my foam false wall pieces slightly off the wall. This way I can easily run my tubing, but also I wanted to avoid screwing directly into the wood to leave a little air (and further eliminate mold issues....I will be installing a low-speed fan to move the air behind these walls at a later point)
If you are planning to screw directly into your main wood frame, realize that you must either a) PREDRILL all your holes BEFORE you epoxy OR b) use neoprene washers/etc (personally I don't know how much/long i'd trust these, but again I overbuilt mine). If you epoxy first, and then drill your foam wall pieces to the wood without any water-protection, you will be drilling a hole through your epoxy layer and into the unguarded wood....water droplets will be able to seep into this eventually causing rot/mold/etc.
I have also run an extra layer of plastic behind my false wall units and have all the plastic collect onto a plastic drip tray that sits on the wood (so the wood doesn't directly get wet pretty much ever).
On my wood itself I used 2 coats of WEATHERPROOF WOOD STAIN and on top of that 2 coats of DRYLOK (only on the inside where water will be). In hindsight, I should have gotten the epoxy you linked and I would recommend using that (and I personally will use it over the drylok for future builds).
EDIT: I know my pics are a bit out of date, I've been working so much on building I haven't bothered to upload my pics; I'll throw some new ones up soon if it'll be of help to you, otherwise i"m prob just going to wait till I'm almost done with the whole build