Oh yes, I went with a rectangle that size, you could go even deeper to get the volume you need to keep a koi or two. Like I said, you could have even more, if you accept the fact that once they get bigger, you may have to give them up.
I actually had 6 Koi in my 1200 gallon pond last year, the biggest one was only like 10 inches long at that point, and I already had plans for an expansion, so it was fine to have that many in there while they were still small.