Great question, Shanthi. I have often have this issue in my area, San Francisco Bay CA, where fog is more common than clear weather. There’s a reddit thread which addresses both the magic and the science around making moon water under clouds. Pretty cool concept!
As for making moon water, it’s pretty simple. Place a container on a window sill, back yard area, anywhere the moon will shine on it throughout the night. Collect it in the morning, seal the container and keep it for a spell that calls for it.
Bright blessings to you.