Add a fan to simulate wind.. It'll force the plant to strengthen the stem.
- However, her stem looks fine, she's spend plenty energy on strengthening it, after you topped it. So don'tworry too much about it.
A fan will still be good to have in your setup tho. They're fairly cheap, and a 15 - 20w should easily be enough to cover a 1.5m2 - 2m2 room.