There used to be an unmarked campsite just north of the Big Bend Tunnel in the Ash Heap. It appears the devs have entirely scrapped it at some point, I have no idea when, but the wiki hasn't been updated.

If you go there now you'll see a clearing of ground that is devoid of bushes and trees and is pretty flat. Not completely flat but enough that I think people might want to build here.

https://fallout.fandom.com/wiki/Big_Bend_Tunnel_campsite

Also, if you keep going south from the Big Bend tunnel spawn and cross the street, you'll find a dirt road with a jeep and army skeleton. Keep following the road and you'll find another good flat-ish spot. It's a split level fenced in construction site that is pretty empty except for some exterior fencing, a half-buried semi-truck and construction crane. There's a lot of open room to build there.