I built the octagon picnic table, and modified in a couple of ways.
1) Due to project deadline (Christmas) (and cost) I opted for composite decking for a finished top and bench, and pressure-treated 2x framing stock. Builder beware! Composite decking can be 5 1/4" wide, which impacts the spacing between the pieces, which means re-calculating most of the table-top pieces. Also, I added lateral structure underneath the benches (more weight!)
2) I really wanted that hole in the center, and hope that my solution (which slightly weakens the overall structural integrity) withstands the test of time.