I like what you've done there, kafer65 -- I'm thinking about getting some Lexan now! So no overheating issues if you leave those little slits open?
I forgot to mention/show that there is a rectangular hole in the upper center of my lower block, behind the license plate. Maybe I'll start treating myself to a very low level of warm air from the defroster when the temp gauge gets to the mid level, just to be on the safe side.
I hadn't been super worried about it before, as it seems like there are lots of other ways for fresh air to come around to the front of the radiator on this car. There are decent gaps around the headlights, between the grille and hood, to the sides of the radiator, underneath...