Here what we went with:
https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1
It's just a 4" queen size memory foam topper. I didn't even have to trim it to fit because it squishes in so easily. I was concerned about it not having any base foam to it and it feeling like you're bottoming out on it, but it's totally fine for me. I like the feeling of sinking into it a lot more than the stock mattress which was very stiff.