Okay, the magnets won't be in until Wednesday but got the 3D printed D-pad installed. Tracked down my old XBox Pro controller, fit the metal D-pad (D-dish?) onto it, and gave it a short test running around the map in Sea of Stars...
If you have a stock D-pad on the Legion Go, this is definitely a must have. The diagonals now exist and the D-pad is nice to use now. Is it the greatest? No, but it's about a 1000x better than it was.
The 3D print was found here: https://www.thingiverse.com/thing:6525043