Next up, simple to do and likely an improvement: partial grille block.
You may have noticed I had Car sporting some nice cardboard-and-duct-tape in the last pic; I proof-of-concepted a grille block before I worked on the skirts, but I got the skirts complete while the cardboard was still on, so I posted them first.
Car has quite a bit of area open in the front, some of it completely useless.
After a tip from RobertSmalls, and also looking at other posts on here, I knew that the top openings were the ones to block off completely. I did a quick patch job with cardboard and plenty of duct tape.
After sustained highway driving on a warm, sunny summer's day, I was stopped 30 seconds at a light on an off-ramp before the radiator fans kicked in (and of course the thermostat needle didn't budge), so I'm fairly confident I left enough of the bottom vent open (basically all of it: what I covered on the bottom was the non-functional parts).
I'm definitely going to block the middle and top radiator vents, and it will be pretty easy to do (hook the top over the back of that plastic grille, screw the bottom into the license plate mount (my resident state of PA doesn't require a front plate)). I do have a bit of a question about the bottom, though.
The bottom lip angles in in the front, so the center vent is the furthest out.
The tempting way to do it is the way I have it in the cardboard mockup, just follow the slant from the upper lip to the bottom. (I'll also duct the inner sides of the opening when I do that, unlike the cardboard)
I worry that this will funnel air under the car, though I still think a tapered angle pushing air down is better than the useless parachutes that the edges of the bottom vent are right now.
Another option I'm thinking of is a short, vertical air dam extending from the furthest-point-forward part of the bumper, down level with the bottom lip, and ducting a radiator-wide, one-quarter-high-as-the-radiator opening in to the radiator. It'd be short enough to not even need support bars, just screw it to plastic bumper.
Still another option is doing a full air dam, extending vertically from the tip of the bumper down to the level of the lowest protrusion of the underbody. I worry about the lack of clearance with this option, though.
Car has pretty high ground clearance for a sedan, I seem to remember reading somewhere on here that high-ground-clearance cars should avoid big air dams?
Any and all feedback welcome. I've got about three weeks before the return almost-cross-country trip.