I would try to repair it even without reassembling/reupholstering - take your harboard or plywood, cut a new seat, take several dowel pins, a layer of fiberglass cloth and epoxy glue. Make holes for dowel pins both in the original seat and in the new one, put epoxy-soaked fiberglass cloth in between (dowel pins should be also covered with glue), hold it under weight for 1 day and the chair will outlast the other furniture in your home. When finished, cut the dowel pins that are sticking out, sand paper the surface and spray/paint with something protective. It sounds like it's a long process, but it is not :)