I bought a flag pole stake on amazon for $13. I haven't tested it out yet but it looks promising. It's HUGE so it should hold it down solidly.

Heavy-Duty 17" Pole Holder Anchor Stake - Flag, Umbrella, Fishing Rod - Non-Rusting Galvanized Steel

https://www.amazon.com/dp/B003FHCCXS/ref=cm_sw_r_awd_cwZAvb1GMV1A3