I have one on an A2, I honestly don't think you'd see one on an A3 as the M320 is more likely seen with the A3.
I could easily be wrong tho & certainly not a critique on your build.
I'm sure the satisfaction is to be able to print your own but these are available...
https://www.sixmm.com/en-gb/collections/stock/stock-adapter?srsltid=AfmBOopkgS4Yx2H_e2KWg9s3eySIPu3D5ez-LrmAeACnsBbxItx47eoA...