Do you notice a difference in acceleration or better handling with the reduced weight? 50KG is quite a lot, but I honestly don't notice much of a difference between just me driving, and driving with my wife in the passenger seat.
I just made an upper grill block out of white coroplast, and it looks good and is holding up well.
I've left the top gap alone since it wasn't as easy to wedge the coroplast in there, and there needs to be easy access to the engine.
Half of the lower grill has been crudely blocked with pipe insulation. I might block the lower section off as well, since temps haven't been going above 185F.