Simple construction for trunk-or-treat
A simple mast looks somewhat like this:

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  XXX  &lt;- Christmas tree holder


So, I think you could get a few 2x4 and make one.  Then just place the mast through the sunroof going in from the top (2 people).  I think duct tape is your friend.  Where the mast is resting up against the sun roof, just duct tape it in to hold it in place.   Remove tape when finished.

A christmas tree holder could be used to add stability.  Just screw in the bottom of the mast to the holder and then the holder sits in your car.  Optionally the same setup could be used outside if you don't want to put it through the sunroof, just put a blanket on your roof to avoid scratches.  Good luck.