[reeferaddict]

Probably the easiest way would be to construct out of plywood, then line it with thinner acrylic, which you may be able to find real cheap... doesn't matter if it's scratched... then silicone as you would for inside corners. (Silicone wouldn't be used for any strength, just to make it water tight) You can even use lighter plywood if you put ribs or braces down the sides and especially the corners. This way you can drill and plumb until your heart's content, and still be able to carry huge water volumes. I would go shallower... especially at that width... lay out something as a model and try to reach in the centre to the bottom... without a snorkel..

I've been thinking about such systems for down the road someday... the other day I bought a 200G Plastic food grade water reservoir with 2" gate valve on the bottom... don't know what exactly I'm gonna use it for... but one day it will be in the "system" for sure... DO let me know what you end up going with, as I'm always looking for ideas...