Thanks Gary for both the instructional video, and informative technical construction details.
Based on all the factors involved, I suggest that building an FSL antenna is much more challenging than operating the finished product.
The critical FSL tuning is largely overcome by using a 8:1 reduction drive 384p variable capacitor. The tuning sharpness in the video is similar to my 40 inch PVC box loop. Of course the tuning is even sharper down near the bottom of the MW band, e.g. 531 KHz. But I have never had issues re quickly finding the loop resonant signal peak.
Like any loop, the optimum induction distance is a function of RF frequency. At first, I position the portable radio a little too far away from the loop, carefully tune to resonance, then gradually position the radio closer until the loudest signal is obtained. The tuning sharpness tends to be greater with distances marginally too far for the loudest signal.
With the 40" air core loop, all my portable radios receive stronger signal induction when positioned center broadside to the loop. Directly back or front of the wires also works, but the signal level is not as strong.
I gather your backyard 9 foot air core PVC loop is the benchmark antenna for comparing all your other antennas. Is it possible for say a 7 inch FSL to match the 9 foot PVC loop? The benchmark is daytime stable groundwave. But with all the recent metro stay-at-home resultant consumer electrical noise, the bigger 9 foot loop may only produce more signal and noise. Hence the law of diminishing returns kicks in.
Kevin Schanilec's FSL optimization paper states that the FSL antenna can provide the same level of performance as an air-core loop 12 times the size. Hence a 7 inch diameter FSL could be expected to approximately perform to a 7 foot air-core loop. The Belrose equation says the air-core loop will provide more signal. But the equation doesn't factor in all the variables. It is only by trial that the more efficient antenna can be determined.