Bucket lids are great as @chascoadmin mentioned. Here are mine I made in a pinch for large sites, I leave these there. If soil conditions are favorable these are pretty solid with washer and nail/spike.
In most other cases I use my trusty pack of targets like these on smaller sites, but I don’t leave them. Hole in the center for survey rod is nice.
Color/finish will affect precise placement of GCP in processing software. You want as flat/matte as possible. At peak solar hours the bucket targets began to “degrade” as too much light was reflected off the target itself. Next time, flat/matte white bucket lid with flat/matte back paint would be better. I would also clear the vegetation in a radius around target to bare dirt.
In Pix4d, notice how much more white the sensor picked up than actually on target.