Didn't take nearly as long as I expected to get to this point!
Let me say though, he did not like it! He let me put it on him without a problem, and I managed to get him out for a walk, hoping to get him used to it. I should have just let him wear it around the house for a few minutes instead. He tried to get it off with his paws, but I got it just tight enough that he couldn't. So then he started rubbing his head on the ground, and eventually shaking his head. He really didn't like it.
We walked a little ways, him getting turkey treats every few steps! But finally I figured he'd had enough, so I took it off.
My mistake was getting too excited over our progress and taking it to the next step too soon. From now on whenever I put it on, he'll just wear it inside for as long as he can stand it. We'll try that a few times a day, until he gets to a point where he's not trying to take it off anymore.. then we'll start the walks with it!
Still, I'm happy with the progress we made in just 1 day!