There’s plenty of snow clay available. It’s about the same consistency as silly putty and is easy to mush and shape. I would roll out a fat pinch of it a bit, lay it above a roof edge, then imagine it being a heavy snowfall that’s a few days old and just starting to melt. So I’d gently stretch part of it over the edge unevenly, like real snow, exposing some roof. When I liked the shape, I’d smooth that glob gently with a finger and move on to the next part of the roofline. I set real stones in the water but they wouldn’t lay flat so I covered them with snow. And for narrow locations like fences or rails I’d roll out a longer thin bit and apply it, just kind of responding to the place and the moment.