This package is free.
https://assetstore.unity.com/packages/tools/modeling/splinemesh-104989
I have implemented it in MapMagic REALLY BADLY however it was very easy.
You could likely add support for this in super fast time. And get all those "HOW DO I SPAWN A WALL/FENCE PERPENDICULAR TO THE SPLINES" comments immediately. Because this makes that simple with scripts like Sower.
Or indeed just look through his scripts and implement to stuff. It's tidy, short and SUPER helpful.
Okay. Hope it helps.
https://media.discordapp.net/attachments/445598770831294465/882587285420392518/unknown.png?width=1115&height=702
https://media.discordapp.net/attachments/445598770831294465/881669371628486687/unknown.png?width=1243&height=702
https://media.discordapp.net/attachments/445598770831294465/882597558617251930/unknown.png?width=1157&height=702