I came back the next day, yesterday, to help Dave and Annelies finish. Alas, we didn’t quite finish, despite applying 5 big batches and worked for five solid hours.
Dave had excavated some new subsoil and it had much more clay in it, so it took a bit longer to get the batches right; and because we had to add so much sand the batches were huge. We were really missing those extra eight adults from the day before!
Here you can see how Dave had cut bottles and then taped them together to install in the bench back. We used a level and a crusty old stick to get them placed right.