To add more medium around the plant and in essence make her shorter is the correct answer here, but doesn't address the root cause. A stem stretches and gets weak due to the plant not having enough light intensity. Which can be a fine line on seedlings as too much can and will fry them. Judging by your images the light was too far away. You can introduce seedling only light via a cheap desk lamp and cheap LED screw in bulb with the diffuser removed. And use these lights till the seedling takes better form and thickens her stem. This is a very inexpensive approach and something we do even in some large scale grows. I also love hitting my seedlings with some gentle air from the many USB powered tiny fans we have found on Ebay or Amazon. These little fans are intended for baby carriages or even a single person in a car, they can be positioned easily as needed. With these I love to hit my seedlings with just enough air for 2 reasons, one being I have a desk lamp or another seedling light source close to the baby I want to blow off any hot air/vapor from the light and I also want to make my baby sway just a bit, dance with the wind. Causing her to do this will strengthen that stem and fatten it up in no time.