Also spray foam... I can't get enough of this stuff. Its hard to mold it into a definate shape the first time. However it can be cut afterwards. To retexture it, just spray on a little more foam and wipe it around. The foam turns into a liquid again... but after a few minutes it will foam back up slightly giving the overall texture the same as if you didn't cut it.
As E says though, epoxy putty is some amazing stuff.