ok so im guessing i should just make square cake and carve the circle out, right??.. how much fondant should i use to cover something this large.. prob 3-4 inches deep

Unless you have half rounds for that size. Do you have a square that big? I would use sheets put together to do it. 2 12x18s wide and 2 high, so 4 total. This would be about a full sheet size. I barely was able to cover a full sheet 2" high cake with 5+ lbs of fondant, so I'd make sure you have more than that, probably 10 lbs just to be safe.