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The chin spoiler is fine, leave it alone.
What may help would be to temporarily install a rear "box" (top and sides, no back, bottom would be good but optional) to the van. It should extend out from the back of the van as far as you dare (even a foot would help, but the longer, the better). The sides of the box should blend in with the sides of the van just before it curves to the back face, and then taper toward each other slightly (say, 11-12 degrees included angle). The box top should meet the van roof at the forward end, blend with it just before the curve down above the back surface, and tilt downward also at about 12 degrees from the horizontal.
This box would form a type of Kamm-back, a classic low drag design.
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