I've been thinking about how to go about building it for a while, but the whole 'invisible' thing has been a problem. It is supposed to be almost like a mirage of sorts, but the best block I found for it so far has been the Cloud Block from Inventory Pets (But I don't actually know how I got them in the first place, since they aren't craftable, and there are very few of them across the world).
Another one has been the one-way glass blocks, but they would still be visible on at least one side of the tower, I'm pretty sure.
by GentlemanLevi