So as of 2021, it seems like a 50A active diode costs around 20 bucks shipped from Alibaba, and you would need 12 of them for an alternator, which is pretty expensive (not to mention they are bulky).
As an alternative, I found this:
https://www.vishay.com/docs/93804/vs-175bqg030hf4.pdf
0.5V drop is respectable, and the package is compact so you can screw 6 of them onto an aluminum heatsink and hang it off the side of the alternator. 30V max reverse voltage may or may not be a problem, I'm not sure. Maybe some zener diodes to absorb voltage spikes would be a good call.