The construction spray foam is likely to be too rigid after setting. The missing chunk can likely be replaced with a piece of regular foam secured in place with a foam glue. This can be covered with gorilla tape or vinyl. The obvious issue is of what the acceptable esthetics is going to be. Myself I do not care as long as the saddle is comfortable. The longterm outcome is that the saddle lasts forever but it gets ultimately completely covered with the gorilla tape.