Full size mattress for cosleeeping? [ON]

LO is 8 months old and crawling and climbing. He’s about to climb out of his crib any day now so we’ll be transitioning to a floor bed. I’m looking to buy a full size mattress that I can put right on the floor and lay with him as he falls asleep.

I read somewhere that memory foam is not safe for kids this age, especially if they sleep on their stomachs. I looked into Avocado but they’re insanely expensive - hoping to find a more budget friendly alternative!

Also looking for recommendations on what to put under the mattress. Can I just get those ikea slats? It won’t be on the floor forever, just until he’s a bit older and then we’ll raise it a bit on some sort of frame. Right now he’s too young so we want it on the floor.

Thank you for any recommendations!