I've found a different bass trap, this may be the best for my primary resonances 38-50Hz. This is the RPG Modex Corner.
This seems similar in concept to Real Traps Megatraps, but RPG makes them in three different tuned versions, 40Hz, 63Hz, and 80Hz. What I need mostly is *SERIOUS* bass trapping around 42 Hz. The remaining problems are trivial. Funny, the RPG website doesn't mention that the unit comes in 3 tuned versions. These are not, of course, pure Helmholtz resonators. But if the "tuning" optimizes absorption of deep bass (which is really hard) that would be very useful. Most traps don't do much at 40 Hz.
Following DIY discussion on these things, the consensus is there is no good way to predict response, even very knowledgeable designers have to cut and try. Heavier membrane yields lower cutoff frequency.