Mattress topper?

Hello! We’ve been consistent co-sleepers following the SS7. I sleep curled around my daughter and she sleeps on her back and I sort of wrap my arm around her to keep her close. She is 6 months now and is starting to want to roll more. Generally, I keep her in place with the way I hold her and I sleep super lightly so I’m frequently waking when she stirs or moves a little. All that being said, are there any very firm mattress toppers anyone likes? I know that technically no adult mattress will be firm in enough for a baby, it’s basically just for my own piece of mind. TIA.