3D Space Editing
Typing build in console activates an experimental feature where you can fly around and place existing entities wherever you want.
Jump and crouch - fly up and down,
T/G - scrolls through all entities,
Q/E - choosing random snaps for selected entities,
Y/H - moving selected entity up and down,
LMB - placing selected entity.
After you are done placing objects, type build save in the console to create a special script that includes ready-to-use "spawnatpos" coordinates of all your placed entities that can be included in starting script.
Please note: In-game keybinds can conflict with the above commands. Make sure when using 3D Space Editing/build mode that you do not have duplicate key binds.