Many have sealed up the gap between the ceiling panel and the "attic" space, including me. Just took the plastic deflectors off, and cut aluminum duct tape into pieces, and taped the gaps. I found some of the deflectors stuck too high into the actual duct work, so some were trimmed shorter so they didn't restrict air flow.
Someone posted pictures of the work, and I'll try to find some for you.
Every day's a learning experience, though sometimes not what we expected.
2012 Jay Flight 19RD
2016 Ford F150 XLT 2X4 SC 3.5L Eco Max Tow
2010 Tundra TRD DBL Cab (Traded)
2 new fluffy Corgis, Bayley and Stanley