[aerohead]

I looked at it and vote thumbs-down on the rear spoiler.
1) The IONIQ is already on the 'template' contour at its upper separation point.
2) Directing the flow up and over a rear spoiler would only re-accelerate the local flow, destroying the only thing which could provide pressure recovery, a higher base pressure, and lower overall drag.
3) I like the front wheel arch gap-fillers. That's a GO!
4) I like the rear wheel skirts. That's a GO!
5) Any additional paneling you can get away with underneath.
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6) As the Cd comes down you'll need less and less cooling, as with Mercedes' new EQXX.
7) There may be a tiny amount of fruit with more 'Tesla-esque' side mirrors, on very-narrow stalks, at least 80mm away from the body.
8) 'Janky' coroplast wheel covers zip-tied to alloy wheels, or ventilated Dzus-fastened MOONs on steel wheels.
9) I've never seen an IONIQ at rest. If you can tell us where the rear diffuser begins underneath the car, there might be a recommendation for its upsweep angle.
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After all existing OEM features are addressed, you need to consider basjoos' comment about a receiver-hitch-mounted tail.
10) The IONIQ is missing 84-inches of body length.
11) You only have 43% aft-body as it is.
12) Your Length-to-Height ratio is only 3.082:1.
13) In order to approach your target 60-mpg @ 80-mph, you'd actually need a fully-gap-sealed, fully-boat-tailed, tadpole trailer, like BamZipPow's.
14) Any remarkable drag reduction is going to come from elongation. There's no way around it.