I started outside a couple of weeks ago and although my tripod is super steady (old aluminum Gitzo Tele Studex 5-section model that must weigh 20-25 lbs) the wind was moving the board on which the target was pinned. Big light stand but too much "Sail" area. It's always windy here in the middle of the Pacific where the trades blow in after last touching land 3,000 miles northeast in California.
Two light stands with weights might work so positioned as to allow the clamps to attach to each side of the mounting board... also smaller board area around the target. I have a 300 to test in the mix so some distances involved. Wish that Mk3 test wasn't crippled by Canon's software interface.
I can't really whine about this as the opportunity to test this independently on a lens-by-lens basis is terrific.