Does the fairing even remotely fit? As in - do the plastic clamps and/or arms fit in between the fore-aft slats? (regardless of a lip to clamp onto)
Without everything in front of me, this is what I imagined could work:
That image was poached and scribbled on from Rhino Rack's Aussie website for the RWFS/RWFL Pioneer Fairing (typical)
I can't recall if the 5 Series slats have track underneath to run T-bolts, so maybe some extra work would be involved,
but affix the transverse (red) bar under the slats, and bolt the fairing arms to the holes drilled in it.
The red bar may need to be somewhat wider to offset the arm holes to accommodate for fairing arc to match the roof contour.
Might work?
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Build Etc...
If it ain't broke, fix it 'til it is.