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.’
i didn’t used this mnemonic at all. i learned the adjective 丸い (まろい) “round or circular” previously while studying grammar and that’s how i know this kanji. maybe it works for you too.
It can be explained very simply
Let’s say lim n → ∞ = ∞ via:
Which is to say that as our n number increases, we are getting closer and closer to a circle. (We will treat a circle as a ∞-gon/polygon with infinite number of sides)
Now imagine a 9-gon/nonagon which already resembles a circle enough as it is, and imagine the drop as being an extra side which will make it a 10-gon and it’ll look even more like a circle than before! This is a very simple explanation for this kanji.
Nine drops is not a round number. Add one drop and you’ll get a round number.
Well, perhaps more to the point, if you add one to 9, you get 10, and 0 is a circle. 
Uh… ish.
I work in IT, and see the word 丸め (rounding) quite regularly, so it was more natural for me to abstract from the circle. And the nature of the 丸 itself is a concept of regularity and enclosedness, so not focusing on the shape could be beneficial.
I used Youtube Maru to remember this reading, since he is quite a unit (big and round), and his face is round:
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