If I don't care about heat in my car (for now), can I just leave the s-shaped ducts between the fan shroud and the heat exchangers off?

If you do so, you'll suck exhaust fumes into the cabin. Usually not a good idea.

Why don't you just turn the heat off?

It disabled now. No flapper boxes or ducts. I just was wondering about the fan shroud to exchanger part. Should I cap off the exit holes in the shroud, or just leave open.

I don't think there should be any real problem as the heat exchangers do let the air back out in either the open or closed position.

BTW, a spray paint can top fits over the heat ducts in the body perfectly. Just use a hose clamp to hold them in place and you don't have to worry about any fumes coming up through the heat ducts into the car.