Memory foam is the way to go. More on reddit.com

https://www.boring.co/products/topper?variant=47949727301915 I think Energex is incredibly hard to beat and they have the firmness you'd need at the cheapest price. For a cover this one looks decent. Haven't tried it but it has enough % of viscose to actually not feel like polyester. https://www.amazon.com/Twin-Mattress-Memory-Topper-Cover/dp/B091Y4Z5BQ/ref=sr_1_7?crid=2MWLMYT4SFJ0U&dib=eyJ2IjoiMSJ9.VugTDE0ZjItlUqK2IDMYlLjyTwJjKdxwzh43yaLGnZaa6jEYU4ZLJ30slfLXVteBiWEAGfaWCivPYVyqYn2lxGBaZYOLeTV26Tuevgb9lUDUgS7B9MHcWpGoxMT7puwxfU0juU2lmqWaQGQaPazTlLL2EnEAeMkblEjSmZdpqEHY4BGwWVS8K_W1DbI1jY4N3vphGdI-Nyl62G_VtfWMXxBgMQ2F2ITaYGaWIQow4hEuYAnS9iLxiEMJvesq2HoviH2z2ZTtw8Y2P2_8v9bH2EQGa-BQlkX1PF7wZFrP2IU.VZC2pA6fK2QD09XCjRYDtZlfQ7ls0kOrgI6tgJYWuR0&dib_tag=se&keywords=topper%2Bcover&qid=1718823500&sprefix=topper%2Bcove%2Caps%2C151&sr=8-7&th=1 More on reddit.com

I'm on the hunt too. Have been for quite some time. Word of advice, don't buy a topper from anywhere that you can't return. I'm the proud owner of a $400 19ILD talalay latex topper that is too firm for me and would love to be able to trade it for a 14ILD because that one might actually work but am not going to fork out another $400 to see. So the search continues... I've bought three toppers from Amazon this week: This one: https://smile.amazon.com/gp/product/B07S83WGMD/ref=ppx_yo_dt_b_asin_title_o02_s01?ie=UTF8&th=1 is a glorified mattress pad. All fluff and not at all as described--I didn't even try it. This one: https://smile.amazon.com/gp/product/B085VRR14L/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&th=1 seems very hot. It was a cool 64 in my bedroom last night and I could feel the heat coming off. No way this would work in the summer. And this one: https://smile.amazon.com/gp/product/B08D1PSY2D/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&th=1 didn't seem to be too hot but was a bit too firm for me. So the search continues. I'm thankful you posted this the way you did and are getting responses. I've tried in the past and it didn't get much traction so hopefully this will glean good info. Good luck in your search! More on reddit.com

Since your existing mattress is sagging in the middle, a topper isn’t going to fill that sag in. BUT, you can put some folded blankets in the saggy valley, then cover with your topper. Another option is to put the topper directly onto the floor and sleep there. One more option is to get a 4x8 plywood sheet (1/4”) to place under the mattress you have, if you currently just have wood slats holding your mattress. But, that’s more money spent, on a maybe solution. More on reddit.com