If you can leave the closet open a bit -
I took a sheet of clear plastic, a few small nails, and covered the top half of the cupboard, so around canopy level to the roof of it was sealed in, even with the doors open. The doors can close snuggly on top of and against the plastic.
Then up the top of that sheet of plastic, I cut a hole for an exhaust fan to poke out. The passive intake like this draws air in from just around the canopy, which flows up over the light, and out the slightly open left door. The hole in the plastic for the fan is in the middle but slightly to the left.
Then I took an old towel and used small nails to hang it covering the left side of the closet, from the top of the closet, hanging down to about 20cm from the floor of it.
I cut a hole in the towel for the exhaust to poke through it too.
Now I can fully close the right door, and almost close the left door.
The air is being pulled in (passively) from right down the bottom, and going out right up the top.
The towel helps reduce light leaks (although I wouldn't flower in there), and I could increase that by adding a second, darker towel on top of the white one.
I can lift the towel up and stash it on top of the closet when accessing plants.