Now that I think about it, 60Hz hum usually shows at 120Hz, wher eyou have a strong T=0 peak. A leg like this one is a strong resonance. However, your spectrograph would benefit from a wider SPL range. 89-49 only covers 40dB, and below 100Hz, you're starting level is about 80dB, so you've displayed a -30dB decay. Starting louder won't hurt either, but I recommend at least a 50dB range on the plot if room noise floor supports it.
What you've read about Helmholtz resonators is true, and that's what makes them well suited to your needs. I used formulas from Everest
. Page 220 has an example similar to your case.
The idea is that a narrow resonance responds best to a tuned trap, while general low bass issues (which you may have truncated) respond to a less-tuned, broader-band device. For these purposes, please understand that you don't want a true, broadband absorber as you have no need for absorption above ~100Hz that I can see.
I based the chart on a 50Hz design center, so I could estimate the performance difference achievable from minor design changes. As you can see, port length is a fine adjust; it's port diameter that drives the tuning frequency once the box volume is set. I'd put the port in front of a sub, to drive a resonance, and do a frequency sweep with a mic in the port, and no absorptive material in the box
. Maximum output should be tuning frequency. Once tuned, I'd stuff it with fill to reduce re-raditaion. Then I'd place it where the desired resonance was strong.
And to be fair to you, this is all planning; I've built and tested nothing along these lines. .