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I'm thinking what might work well for belly pans is to go from the front to just ahead of the bottom of the firewall, then on the next section curve it up to fit to the bottom of the firewall/floorpan area, cutting as needed to fit closely around stuff.
Then you'd have an outlet for engine compartment air through a slot that shapes and smooths the airflow, and as little as possible of that hot air getting between the floor and the rest of the belly pan.
Google semi trailer skirts for plenty of interesting reading. Just about every semi trailer now is running various types of angled skirts. They catch the air coming from under the tractor and in from the sides, smoothing it out and directing it out past the trailer wheels. Around 7% MPG improvement. Full skirts that resemble "ground effects" kits and also skirt the rear wheels do even better but cost more, are more difficult to install and have to be partially removed for wheel service. They'll also get trashed by tread coming off and blowouts. I suspect the extra cost and the annoyance/time/damage potential of full trailer skirts are why they're seldom if ever used.
IIRC the idea is a big isosceles triangle wedge with a 15 degree pointy end with the back ends right out to the edge of the trailer. The upward angled cut at the front also factored in, the angle works better than a simply straight bottom edge. Might be an experiment to try under your van.
One article I found when I looked this stuff up was a paper on the effect of various rear mud flap designs. A full width flap increased drag and reduced MPG but one with a gap in the middle increased MPG over no flap at all. Have to find the article to see what the numbers etc. are.
What's the average MPG for semi trucks these days? I recall not long before the flat front tractors died out, some company built one with every aero trick they could come up with and squeezed 10 MPG out of it, which was bleeping excellent for any semi.
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