What object would Voldemort have made his 6th horcrux object when he first tried to kill Harry?

In Half blood prince, Dumbledore suspects that Voldemort was going to make his final/6th horcrux when he was going to do the significant killing of Harry/stopping the prophecy that he'd just been told by Snape. Harry himself ended up being the horcrux, but what was the object Voldemort was going to use, do you think? He thought to use Nagini as the '6th' one later, it seems.

My personal theory is he was going to put it in his wand - a sentimental and powerful object, but after becoming a powerless "spectre" with the rebounded killing curse, decided that it shouldn't be an object always on his person, maybe? Would've strengthened his belief to not use his wand as well after the wand connection in the cemetery and finding out about the twin cores from Olivander. (Edit: and make him more obsessive about finding the elder wand/getting rid of his own wand.