It’s crucial, that you have exact plan. Our greenhouse will stand on 18 point foundations. I bought 9 concrete channels (diameter 30cm) and cut them in half. Concrete channels will lay on bed of gravel groats and will be filled with concrete.
All foundations were water leveled and diagonals double checked. It was one day work and another day to mix concrete and built it in channels.
Hint: it is possible to buy concrete channels 100 cm or 50 cm long. 100 cm cost 10.60€ and 50 cm is 8.78€. With little math and one hour of work I saved 62€ 🙂