Although I haven't got a 370 board up and running yet (one AMD board waiting for a psu at the moment), I have got an m.2 adaptor working on my Z270 board. Its using a Lattcure M.2 adaptor. It provides a 1x pcie slot just like the mobos use and its working great. Doesnt need the external power attached either. I will be using them on my X370 build soon and can report back how it goes. But the Lattcure ones are definitely great quality.
thanks, i think i'm going to try that pcie 1x m.2 also. The one with just USB port not working on both 370-a or -p model