Many of us cut up one of these Ikea 3-7/8" thick foam mattresses. This makes a good seat cushion and is comfortable to sleep on.
MINNESUND Foam mattress, firm/white, Queen - IKEA
MINNESUND Foam mattress, firm/white, Queen A good choice for your day-bed or child's extendable bed. It's perfectly dimensioned and the all-over support ensures a comfy, restful sleep even for those who'd rather stay up and play. Easy to bring home since the mattress is roll packed.