Try the Logitech C270 (or C920, but that one will cost more than a SHIELD Spot) webcam as a cheap mic solution. Both have drivers built into the Android TV OS itself.

I use the C270 on my SHIELD for the mic alone seeing as I use my Logitech Harmony remote with it, which doesn't have a mic. Works well (it's not always on/listening, just speak normally when you hit the mic/voice button on screen - I obviously have no clue if it will work with always listening when Google Assistant drops).

I have the same model, and i use Samson Go Mic

you can just use a smartphone to do that and sony also sells a remote that has a microphone in it some of there TV's actually come with it