Firmest dog bed available on the market?

Our dog is very much a floor dog. We've spent decent money on beds and cots, but he always ends up on the floor by choice. He also refuses to get on and stay on the sofa and human bed. So, we guess that he doesn't like surfaces that sink beneath his feet.

Recently, we spent a couple nights in an Airbnb with a very firm mattress. He not only got on all by himself, but also stayed and fell asleep on it.

So now we're on a mission to find the firmest dog bed possible. If you have any recommendations, we really appreciate it.

Thanks!!