I just tried tying it up and the fishing line just slides down the stems and doesn't stay, Sword stems are pretty sturdy and forceful.Amazon's will grow huge. I think tying it up is a great idea, but eventually I think it'll find a way to spread out.
You can always trim it back. Wait until it has a few smaller leaves on it, and then start trimming the longer/taller leaves off. It'll continue to grow, and you'll be able to knock it back down in size.
I eventually just had to get rid of mine, far too big for 45g.