I have never built one but if I was to build one here is what I would do...
First off I would set the tank on a piece of plywood, trace the shape of the tank. That would be a template for the bottom, repeat the process for your canopy top. depending on how you want it to look you can increase the size of the bottom by 3/4 in on all sides which would allow you to set the piece of wood on top of your sides instead of having your sides butt the bottom.
To figure your angles pick up a angle finder from home depot, then you can determine your angles for all side pieces and cut out on a table saw.
Repeat the process for the canopy. I would assemble the canopy so that the top was hinged or a couple of side pieces were hinged for access to the tank.
Then add trim pieces to make it look nicer.
Stain/Varnish or paint to suit your taste.