Depends on what you want. We ordered an inexpensive 3” $100.00 memory foam
Mattress topper from Walmart (or anywhere). Made a template with heavy paper,of the OEM mattress (terrible btw) cut the memory foam mattress to fit, threw on top of OEM, covered with a sheet. Done! Memory foam cuts like butter with a sharp carving knife. Check YouTube.
I’ve never met a OEM mattress worth a plugged nickel and that goes for $$$$million yachts.
BTW, easy to roll up and out of the way if you have to access under the bed.