I don't know if this would work for you, but I thought i 'd share our sandbox set up. We had a very organic, sort of kidney shape, large hole dug for a water garden, but didn't end up doing the water garden, so the space was open for another use! You can find large plastic liners at garden stores/home depot, etc. to create a template and then use the liner for the inside of your sandbox. We simply lined ours with two tarps. We then had a truckload of dirt brought in and dumped. We surrounded the sandbox with large flat rocks which gave it a nice organic look, and also allows for a place to sit, or to use as a resting spacee for little cars, trucks, etc. We try to remember to cover with a tarp but it doesn't always happen. Really our only cost was the dirt as we had old tarps. We're lucky in that the sandbox is shaded by birches and flowering trees, but I have seen others use sailcloth and poles to create shade.
The great thing about the sandbox is that it's huge. We can easily have multiple kids in there. The big space is so satisfying because they can really dig and build. We hope this summer to be able to run some water out to the sandbox with a hand pump because there is nothing better than water and sand together according to my kids!