I know that it is difficult when you ask a question like this on a message board that spans the entire country and beyond.
It really isn't rocket science although it can certainly seem that way when you are handling one that won't eat or perform the way you want him to.
In essence, it REALLY is about calorie intake. You are in Minnesota, so corn is the easiest source of calories that you have readily available to you. The 3% rule (in my opinion) is a good one.
If I were you, I would search Steerplanet for the AAOK Feed Ration. From there, I would send a PM to AAOK, tell him which of the ingredients you can NOT get, and then have him help you with it. This ration is pretty widely used, and seems to work really well.
I don't disagree with the advice you are getting in this post. I just feel that it is important that you are careful adding in a little of this here and there. Next thing you know, you may have a ration that is so far out of balance that it will be nearly impossible to get any results or get the steers to eat it.
I also agree that some of the middle expansion comes with time and weight. In my experience, a shallow middled steer has bigger problems than his appearance. Typically they don't gain very well for us. Stretching out their belly with beet pulp or [insert roughage here], will only change their appearance. It won't necessarily make them perform like a naturally deep bellied steer.
So, back to answering your question which is how to get him fat and get some belly on him......feed the ever living life out of him! Push that dude as hard as he will let you!
That is my input, and I am by no means an expert. When it comes to rations, I don't mess with them. I leave it to the experts.