I decided to make some ground control points (GCP). I was able to find some cheap vinyl flooring with a black and white checkerboard pattern.
I bought the vinyl flooring at my local Lowe's store, named "Tarkett Berkshire 6-ft W Black and White Tile Low-Gloss Finis Sheet Vinyl." It was $5.94 per linear foot or $0.99 per square foot (six feet wide). Any type of black and white checkerboard pattern would work if the squares are the right size and it's not too glossy. You don't want it reflecting the light. The vinyl flooring I bought is not quite flat, which would be best, but it's not so reflective that it might be a problem. (Or so I hope, I haven't used them yet.)
The GCP consists of two black squares and two white squares, each six inches square to make one GCP at one square foot. It should be the perfect size if you're not flying too high. I glued the vinyl floor to some scrap half inch plywood I had that I cut into one foot squares. I used General Use Multi-Floor Adhesive, standard stuff for vinyl floors. That was only $4.98 for a quart. Don't get it on too thick, less is best.
I'll also drill two holes in the corner of the white sections for nails. I don't want the GCP to shift on me, so I will nail down the GCP for the time that it is needed in the field.
I stacked the GCPs on top of each other sandwiched between a sheet of newspaper with weight on top for 12 hours as they dried. Be careful if you do this, I somehow had a couple of them shift on me.
With the glue and spreader, I was able to make 18 GCPs for just over $25. If you need to buy the plywood, I guess it would be another $20. I used half inch plywood, I suppose I could have used thinner plywood. 18 of them are heavy, but I did want to make the GCPs strong so they would not flex.
Attached are some pictures.