two of each, drains and returns is a good plan, this way if one should get clogged for some reason, the other is still available.
as far as supplying power for your canopy from inside the tank, I think that would make it a bit difficult to remove the canopy. how about running the electric cord for the lights up along the outside-back of your tank, in the middle where your strainer weir is covering it. use some of those little stick-on cable clips, or some of that Panduit wire raceway.
using a rubber made container for a sump is a good idea - the only thing you will want to do it build it some sort of support to keep the sides from bulging out when it's full of water. The lid would help with this, but only if it remained intact... then you wouldn't be able to remove it without draining the water down.
also having the sump covered and the tank covered gives you no place for degasing and o2
absorbption. If it's going to be a heavily planted fw
tank, then no degasing is needed, the plants will do it for you, but saltwater or fw
fish only need a way to bring in fresh air. also, evaporation is the easiest and cheapest way to cool your water... a fan on the sump accomplishes this real well.