The current mental image for 丸, using the nine and drop radicals, is ‘What’s something with nine sides? Just add a drop and you’ll have something that’s almost a circle.’ Now I know it’s hard to come up with memorable images for thousands of entries, but that one’s pushing it for me.
I find I can remember it better as ‘You take the number 9 and you drop it, and oh no, the drop broke off! What are you left with? A circle.’