The reason for blocking off the area above the fan is so that the air doesn’t just loop around in a big circle. The fan will push air out, but that air can loop around the open area at the top and go into the back of the fan. The fan would just be churning the air right around where the fan is. If you have blinds, have them come down just to the top edge of the fan so that they block off the top part. The bottom part with the fan would not have blinds covering it. That will help prevent the air from circulating around the top. And make sure there’s no open space on the sides of the fan for the same reason.
I would think the box fans would be a pretty good experiment for those window fans. The window fans may have a little more force and may work better, but I would think the box fans wouldn’t be too bad. One thing to make sure is that you have a way for the air to come into the room. If your box fans are pushing air out, you need an opening somewhere for new air to come into the room. Leaving your door cracked open may work for this. Or if you have windows on two sides, have fans on one side blowing into the room and fans on the other side blowing out. And keep in mind that the wind outside will also play a factor. If your fans are blowing air into the wind, the wind will push back and the fans won’t be very effective. You would want to orient your fans so that they push in the same direction with the wind to maximize airflow. That may mean that some days you have the fans blowing into the room and some days blowing out.