Can someone point to "the shade nut" with a real life picture? I'm assuming it's not a shade ring.
I would fit a disk of aluminum foil on top, either over or under the shade nut. You could also use sheet metal, such as from the bottom of a coffee can. This disk should not extend all the way out to the shade wall--leave at least 1/2" for ventilation to prevent dangerous heat buildup. Do not use tape or other combustible materials. If you put the disk under the shade nut you could also use a standard or fender washer to prevent damage to the foil.
Is it this? See my red lines.