Here's a pic of my '97 HX grill block. Sorry the opening is on the far side away from camera, but you can see it goes right up to the license plate. This was an early photo of it. Now has an "L" shaped leg added that extends below the license plate, towards the center of grill. Also I extended it on top, bottom and outside edge, about 1/4 to 1/2 inch bigger on each of those cuts. I needed that in summer for sufficient cooling. Upper grill is blocked but only on driver's side, radiator is on the other side.
In winter, I used to block up the lower grill openings with foam pipe insulation between the slats. I expect to cover the opening you see completely from about December - February. I found in winter I could block all the lower and just leave the upper open. I watch engine temp on the ScanGauge so I see what the temperature is doing.
In my 6th generation Civic the radiator fan is on passenger side and a/c condenser and catalytic converter are on driver's side. So for cooling you only need to open the passenger side. If you block the a/c side and run the a/c, the a/c condenser fan comes on and will blow air through there. Mine worked great for 3.5 years and finally the compressor froze up but I don't think that was due to the grill block.