To start of - I failed big!
Yesterday I took all the measurements, made a neat blueprint - but mixed up the length with the width while making it, resulting in me building it wrong It is fixable but it will be 10 shorter on the width and 8 less liters, unless I start unscrewing stuff..
Anyhow, here is how it looks atm! Made out of marine plywood:
Attachment 6603Attachment 6604
** Update **
The whole thing is now done and I'm waiting for the silicone to dry up, and then one more night to check that it holds water.
I used 3 layers of Plasti dip on the inside, which is a type of liquid rubber that comes in spray cans. It has been used by others in aquarium builds with no problems, and someone wrote that the company who makes it claims that its perfectly safe as soon as it has cured. We put silicone on the plywood before coating with the rubber tho, and the rubber didnt stick to the silicone so we put another layer on top just to make sure.
The outside has two coats of outdoor paint, which will have no contact with the water.
What you'll see in the pics is the tank, and the box which will hold the majority of the plants. The upper level will be dwarf mondo grass (aka Kyoto grass) and some mixed plants in the big box.