Yep these are
1) Open: /plugins/terraincontrol/worlds/world/gridmap.png
2) check for 4x4 pixel single color squares. ( these mostly are the custom structures )
3) then follow the horizontal white line to the right and note the number ( something like 1000,0,0 )
4) go back to the 4x4 square or 3x3 or 5x5 then follow the vertical line ( like 0,0,4000 )
5) combine the numbers and add 200 in the middle like: 1000,200,4000 those are the coordinates of the white cross grid.
6) type: tppos 1000,200,4000 and fly down. The custom structure should be near!
Im currently on my mobile phone so im sorry for the vague directions. If you still fail to find a custom structure let me know and ill help you out when i got some time to respond with a clear mini tutorial
Thanks for letting me realize this should get a tutorial by the way!
Those annoying windmils keep clipping, they should only do this rarely. I will try to tweak them even more to avoid any clipping, added to the to do list for the next build ( Current build is scripting DLC )