recommendation for mattress pad/cover for the stock mattress

I got a 4" Topper from Wally World. It improved things but putting a soft top onto an already squishy mattress didn't seem logical.

I went to Lowe's and got a sheet of "luan" plywood - the really thin (1/4") and lightweight stuff. Sliding the 'plywood' between the original mattress and the Wally World topper has made a big difference.