Hi Steve,
For the speaker calibration measurements what I initially tried was placing large sound-absorbent panels behind and to the sides of the measurement mic, with each speaker having been moved to the center of the room for purposes of that measurement. The panels were placed something like one or two feet from the mic. That did NOT provide good results, because reflections from the panels themselves, while small in amplitude, were so close in arrival time time to the direct sound that the "booth" did more harm than good.
I then placed the panels against the nearest reflective surfaces. One being a stone fireplace on the wall on one side, and the other being a large piece of furniture on the other side. That was definitely worthwhile in my case, as the room is only 13 feet wide and the piece of furniture (actually an antique radio/phono console) extends out about 2.5 feet from the wall on that side.
In your case whether doing something similar would be worthwhile presumably depends on the distance to the nearest walls or other large surfaces, and their reflectivity. Perhaps consider trying it initially without any such measures, and see on the resulting impulse response/time-domain plots how many milliseconds from the direct sound arrivals you can "window" the measurements, before reflections become prominent.
In my case, if I recall correctly the duration of the "window" I applied to the measurements was limited by reflections from the ceiling, occurring about 8 ms after the direct sound arrival. (Reflections from the floor were not significant because in addition to it being covered with a thick rug, when making the measurements I had placed a pillow on it, directly in front of the speakers).
The panels I used were these:
https://www.bhphotovideo.com/c/product/401266-REG/ClearSonic_S5_2_S5_2_Dark_Grey_SORBER.htmlGood luck! Regards,
-- Al