From The Indymotorspeedway site:
The last driver to have his likeness placed on the original trophy was Bobby Rahal in 1986, as all the squares had been filled. A new base was added in 1987, and it was filled to capacity following Gil de Ferrans victory in 2003. For 2004, Borg-Warner commissioned a new base that will not be filled to capacity until 2034.
Since 1988, an 18-inch tall replica of the trophy, a Baby Borg, has been crafted in sterling silver for presentation to the champion. A new tradition began with the 1997 winner as BorgWarner also presented the winning car owner with a Baby Borg.