I've been thinking about making a grill block, & took a good look at my grill tonight. Looking at this picture, what more do I need? Am I trying to make the space flush with the front of the car, or is this OK?
I have the same stock grill block as you, I then added a flush grill block (check my garage) that is simply caulked in to place, I did it mostly to make the opening even smaller, so now it only leaves the last set of slats open, and in 105 degree weather I didn't have any problem over heating.
I beleave you are correct in the buffer of air idea, most of the air drage on a vehicle is at the rear end where it is coming back together as it flows around the vehicle, but what the grill block really helps with is reducing the amount of air going in to the engine compartment, as that air has to exit at some point, and it can't flow in a straight line, as the engine is in the way.
On my '95 Civic DX, I didn't make it flush. I used some old real estate coroplast signs, cut them into three pieces (because one sign or two signs weren't big enough to make one or two big pieces), and used tie wraps to secure them to the grill. I also blocked the small grills on both sides of the large center grill.
Wonder if you could just get some Suzuki Swift headlights to fill that gap up. The Swit, at least in Europe, never had those recessed lights. Here is a pic of a 1993 Suzuki Swift for sale in the Netherlands:
my mileage goes up and down so much that it's hard to get a good reading to tell what works, and what doesn't, sorry.
I need to update my gas log, but I try to keep notes of things like doing the grill block.