Hi Sue,
I see you put this question in awhile ago but I'll put this answer in to help others with the same problem.
I would go to a place that sells and cuts wood for you.
meaure the area under the cushions and go there and have a piece of 1/4 inch to 1/2 inch depending on the wieght of the individual or individuals going to be seated on it use plywood or particle boad(mdf ??) and have it cut to size,not as wide as the cushions you dont want the back of your knees cutting into the edge of the wood just were your buttocks will be meeting it plus a little and I think this will solve your problem.Better than buying a new sofa or chair. Hope this helps.
Peace be with you. Live long and prosper
George
George ''Legion'' in the game rooms, July 2010
