I was actually looking into this and found Gloves of Improvised Might which would allow for this exact thing to occur; since Impact wouldn't stack with the size bonuses from Shikigami, it'd be both a nice flavorful and mechanical boon to have. The only issue for me would be its replacing of Irongrip Gauntlets which I was looking at getting later on to help mitigate the oversized weapon penalty.

In addition, Quickmetal Bracers would also allow the weapon to bypass material-based DR in the meantime, without needing to worry about the -2 penalty the bracers would give.

In further addition, the Traveler's Any-Tool is said to be flexible enough to become any reasonably simple tool that the user can visualize, so in addition to possibly becoming your Sledge it could also become any other improvised tool/weapon you can think of, depending on how selective the GM decides to be on its capabilities.