I would rasp, better to carve, the upper edge of the perimeter to be a more natural slope. I'd resort, otherwise, and to give up a bit of the area inside that confine, to using spackle, Durham's Water Putty, or something like that, to craft a slope and to save me the trouble of carving out all that foam. This treatment would add immensely, at the same time, to 'water-proofing' or sealing the edges of the pond once you pour.
I'd paint the middle third a dark grey-blue or black, maybe a dark teal colour, and then lighten it right up to the edge.
Mix and pour about 1 cup of two-part epoxy (Envirotex, or see your local hardware store for a 'finish quality' epoxy), cover it with clean cardboard, and let it set for 24 hours (IF...you mixed it according to instructions). If it bubbles, which it almost certainly will, use a soda straw in your mouth to blow across the surface. They'll all disappear like magic. Wait ten minutes first...they often disappear on their own.
Next day, if you want it deeper, or don't like the painting effect you rendered, paint over the first pour and repour. If you just want more depth, same thing, add a layer. Note that you CAN tint these epoxies using cheap acrylic craft paints. A drop or two, not two or three blended squirts!!! Also, if you want it a little turbid, a small pinch of plaster of Paris powder.
When that second, or third pour is done and cured, slather a layer of Mod Podge Gloss Medium over the entire surface and turn your applicator foamy on its side and stipple the entire surface. Allow at least two full days, often more for the highest/thickest peaks, to dry. I do prefer gel gloss medium because it stays put, but you'll have to go to a craft store for that..?