The bench truck seat developed sinkholes in the driver's side seat base and back cushions.  I purchased a $5 firm foam sleeping bag pad and trimmed pieces from it to fit the outlines of the seat base and back for cover pads.  I built up the "sinkhole" areas by cutting more pieces of the pad to fit the "sinkholes" and taping them to the cover pads using Gorilla tape.  I looped the tape to stick the flat pad surfaces together, and taped the edges, too.  It took less than an hour to make these pads to build up the seat and make it comfortable to sit on again.  I made a fitted cover out of outdoor furniture fabric.  Seat fixed for less than $22 versus new replacement seat at $600.