Alright, here's a comparison. I pulled the OP's picture and lined it up with two pictures pulled from google images (apologies to whoever owns the pics). Fledgling/juvenile Hawk Owl on the right, fledgling/juvenile ESOW on the left. The differences in structure and plumage should now be obvious.
On structure alone you can eliminate all the larger owls. The only potential confusion species for this location would be the small owls - other Megascops (none in Wisconsin) and Aegolius, but Saw-whets and Boreal have radically different juvenile plumage.
On plumage, look at the back and wing pattern closely. Dense brown vermiculations, and barring in the flight feathers. Compare that with the plain brown and white spots upper pattern on the hawk owl. Also, even on very young Hawk Owls the distinctive facial pattern is obvious - broad black borders on a white face. The Wisconsin bird has grey-brown vermiculations, like Screech-owls.
The lack of ear tufts is no problem at all. They are less developed on young birds, and they can be sleeked back. If the Wisconsin bird has them sleeked back, given the angle (looking down) you wouldn't even see them standing out against the rest of the crown.
I hope that helps.
Edit: I should add that I think you are right, Steve. The bird in question is more advanced than fledgling, but it's not adult plumage, so it's a juvenile.