[aerohead]

I went ahead and spent some time with the IONIQ blueprint:
1) As a 1st-approximation, the extension to your existing spoiler, beginning @ 1162.6mm from the ground, would fall away @ 16-degrees.
2) @ 12-inches from the 'so-called' diffuser, the curvature of the upper surface would morph to 16.5-degrees inclination, @ an elevation of 1033.3mm from the ground.
3) @ 24-inches ( if using a receiver-hitch-mounted cargo carrier ) the curvature would have drooped to an 18-degree inclination, @ 935.5mm from the ground.
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4) The diffuser would follow the line which exists at the bottom of the rear quarter panels, rising on a 2.8-degree inclination.
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5) The sides of the IONIQ follow different cambers, bottom, beltline, and roof top. Each would follow curved pathways specific to their starting conditions. The most complicated portion of the elongation. The idea is to get it 'right' the first time.
6) See if you can locate: US Patent 8590961, February 21, 2012. Look how Breidenbach handles his boat tail. Your scheme would be similar, but you'd want 'curves', not just flat planar surfaces.