It's not a mod to the actual charger per se. It's literally just an adapter from a 6-50 plug to a 5-15/20 plug. I've linked to an Etsy page where a guy (not me) makes them and sells them mostly just to not steal his picture or credit.
https://www.etsy.com/listing/3843905...level-2-charge
Credit goes to Chris TX on GM-Volt.com for all the initial legwork in early 2016. I believe the Etsy page is his as well.