Don't overthink this. The only thing that really needs to be level is the perimeter where the side posts are placed. The center or ends are often dug deeper to make a deeper pool for diving.
I repaired a JD riding mower hood that was broken into numerous pieces using this tool. It makes a mechanical repair connecting the parts. After doing this process I put fiberglass cloth and resin over the staples. Be sure to use the staples on the underside.
Next I filled the topside with...