3D Space Editing

build - 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.