I put a thick pond liner on top of corrugated eternite (the asbestos kind) then filled up with hydroculture clay pellets (should have used lava) and put a reasonably thick layer of soil on top of it. Seeded some meadow plants and added irrigation when it’s too dry. It insulates in winter, cools down in summer, looks good (yay, flowers) and insects and birds love it. Would definitely recommend, unless your climate is too dry and water is scarce.