Peter,
I just carved my seat last Saturday by using one of those 3M Roto paint strippers for a drill. It made short order of the hollowed out portion of the seat and with a little practice I could get some fairly smooth results but it still needs some finish sanding. At first it was tough using it as it wanted to grab and run along the top of the foam but after awhile getting used to it I was surprised with the results. Other than that, maybe one of those Ginsu knives as seen on TV or one of those army surplus machetes with a nice razor stropped edge might work.