The stock tank pond: scheming on what comes next

Frequent readers may recall that I've been doing battle with marauding raccoons for several years now, they're a constant source of frustration. The plants surrounding the stock tank have also changed things, they've grown and it's a much shadier situation than when I first planted up the tank back in 2014. This photo is from last September...And this one from 2014, when the tank—and the whole northside of the garden—had just been planted. Kinda sparse, eh? (well, except for all those beautiful agaves)
Back then the "pond" was planted up with all sorts of things, and was amazing. Fast forward a few years and the racoons took away all the joy. They discovered it was a source of water for bathing, drinking, playing and digging grubs in the soil (of the water plants), and systematically destroyed anything and everything in the stock tank. Every damn year.
My solution last summer was to put a gunnera in the tall galvanized vent planter I'd sunk into the middle of the tank. It worked as far as keeping the plant from getting destroyed by the raccoons.
But going into this last autumn I decided to not try to overwinter the gunnera, to give it away and come up with a different use for the tank. I was however quite surprised when I pulled the plant out and saw just how big it was!
So, empty tank. Empty tank somewhat hidden by the ginormous Agave ovatifolia in front of it! I'm not complaigning.
What kind of rock to use though? I wish I could get a really thin urbanite, something about 1/3 the normal thickness...what would that even be? Let me know if you can think of something.