You could also buy a waterproof mattress cover (the non-crunchy kind) and remove the part that wraps around the side/edges of the mattress, then sew up the ends, tuck into the cushions and cover with a blankie or something more attractive.
I tuck an inexpensive, but really nice, soft velourish blankie into our couch cushions where the boys hang out. That allows me to wash it frequently and keep things smelling fresh.
__________________ Kristin, Max and Teddy
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