The USB audio dongles arrived.  They weren't CM109's, but showed as simply Generalplus.  They were probably the GPD8106B.  With the mic gain at 0, the audio editor clocked it at -75dB peak noise & -85dB RMS noise.  The worst case for the CM109 was rated at -76dB.  Moreover, the noise wasn't RF.

The Generalplusses were infinitely smaller than the custom solution & the noise would be undetectable, compared to the microphone noise.  The problem with the custom solution is many days passed without an enclosure emerging for it.  The gain control would probably be adjusted only once, but it was a pain to access.  The noise floor would never be reached, in the field, without shielding.

Setting gain with alsa requires 1st getting the card number with 

aplay -l

Then passing the card number to the -c option, along with a bunch of other options.  

amixer -c 2 set 'Mic' playback unmute 13 capture 24 cap

sets the microphone monitoring to 13 & gain to 24