I hacked the crater out of the 'Maadim Vallis' model on the Space Graphics site also using their 'm46 Texture'.The model of Spirit came from the Nasa version of Celestia.If you have an ssc with Spirit already you may want to edit it.You can find a zip of it here
http://www.ibiblio.org/celestia/creators/jestr/Mars%20Surface%20Features%20v1.3.2pre6+.zip
This zip has all my Mars Volcano models also,but will only work in the latest versions of Celestia.Hope you like them,Jestr