When I was ready to build Soggy Dollar’s slab, I got out my book on concrete construction. I remembered that it had some info on constructing plywood forms for pouring gracefully curved edges of slabs, like curved sidewalks, etc.
You can easily bend plywood forms by cutting lots of slots on the outside of the forms. You can then bend the forms into graceful curves, then spiking them in place. This is what I did for the Soggy Dollar slab. Take a look!