MountainPass responded on similar thread on M3OC suggesting they used a Mastercraft socket thats 17.3mm dia. Looks like the thinnest so far on either thread without grinding. Only problem is, they are difficult to fine online.. might be hard to get a hold of.
Snap-On also makes a thin-wall 13mm that's 17.6mm and might be more readily available.
Update: I scoured Amazon and found a 13mm 3-pack which states its "thickness = .6in" or ~15.25 mm. I contacted the seller to see if they can confirm the diameter and will report back.
https://www.amazon.com/13mm-Socket-Pack-Drive-Shallow/dp/B07BQJX3N8/
I'll be visiting the machinist tomorrow to get the sockets I obtained turned down on a lathe. Give us a shout if you'd like one! It's not just for people who bought our coilovers