You're probably safe to block those side grille sections from the rear -- I'd also look at adding weatherstripping around the edge to seal against the bodywork & headlights when installed. (I take it you'd have to remove that to change a light, so caulking isn't an option?)
I wouldn't rule out a partial Kammback/downswept "spoiler", if you've got the fabrication abilities. OEM ones of varying lengths are becoming commonplace on hatchbacks & estates. EG: Honda van, Mitsubishi Mirage.
They're not as conspicuous as they might have once been.