It's just regular powdered citric acid from the baking aisle and distilled water.I believe it’s 1% citric acid. @InTheShed could probably break it down into better formulation for you upside down folk
Here is my recipe (scale up or down as needed):
5g food grade citric acid
500ml distilled water
12 pipette drops dish soap (or if you prefer, some amount of yucca or aloe as a surfactant, though I haven't used them)
Spray the whole plant and pull the buds away from the stems to spray in there too, since a lot of PM grows in the tight spaces.