Just don't even think about it, though I'm sure that the area we're currently harvesting (on the map its above the main outpost which is up and to the left of the giant circle) just isn't gonna have enough sand/sandstone to cut it, we're going to have to start hauling from the desert, which with several people working at one time won't be too bad but I feel like a powered minecart system with miners on one end quarrying and builders on the other end receiving and building would be the most efficient method. In either case the fully finished tower project is gonna be at least a week, and that's if we get several people online for several hours a day, so I'm saying we can have this done in two weeks if we keep the project on the scale we are, allowing for break time/off days and don't use anything other that sandstone for the construction of this thing. That means a ton of iron, which means we need to find diamonds for our tools and stick to leather armor for the time being.
The best way to fill an are with water in is to do it one row of blocks at a time, cut out a line, fill it in with water, dig the next line. Then you can make it deeper easily by just digging it deeper. I've tried in the past digging an area out completely and then trying to fill it and the physics just don't allow it, you have to be able to dump the water up against something. Lava in trenches would take quite a while, and you'l need a lot of buckets and hauling, plus anywhere it connected with water would make a cobblestone ring. I really want to use some of that 4 or so chests of dirt we have and turn that into a floating island fortress somewhere in the north with a big hole cut out of the ground kind of like out current quarry, and have lava flowing out of it in a huge lavafall into the void below it, but that'll take a lot of time and/or dynamite to create the effect I want, sort of a perpetual seas forever rushing in to the maelstrom look.