My first inclination was to frame out and level a form, fill the gaps between the form and the flagstone and pour a slab on top of the flagstone. I had the idea as well to lay down a heavy duty tarp and place the form on top of it, pour the slab and then trim it down after the slab has cured.( to prevent the concrete from ruining the flagstone if I ever want to remove the slab).
What thickness slab would be sufficient in this application? ( spa weights in at 4000-4500lbs, 91"x84")
Any other ideas on how to accomplish this?