I have no experience with those but I have made control stands with similar features. Here are a couple pictures:
It starts with a Baygard plus step-in electric fence post and I cut the top off using a metal cutting blade so the overall length is about 38". I remove one of the adjustable hooks so there is only one left and that will be used to hang the flag. The posts cost me about $2.70 ea. on-line.
Next I epoxy a 1/2" tube to pipe male adapter to the top of the post. These are about $0.60 ea at Home Depot
The SI bracket is screwed to a PVC Tee where the stem of the tee is threaded so it screws onto the adapter that is epoxied to the post. I think the Tees are about $0.60 ea. too.
The control flag is hooked, not tied, onto the ajjustable hook using TWO of the three strings that go from the central point to the edge of the flag. I would get a picture of this in place but the ground is frozen.
As mentioned by AZ, being able to remove the head unit makes storage much easier and it's easy to hold 10 or 12 of these stands in one hand. Of course the head unit needs to be carried but those go in a backpack with the SI units.
The baygard plus post is the only one with adjustable hooks so there is no need to cut off the extras. Also, cutting off the top shortens the post so it doesn't stand as high as most of the fence posts used today at events. Making the overall length 38" will place the bottom of the flag about 12" off the ground when the post is embedded all the way to the step.