A simple mod that adds one item - Structure Marker. This item lets you mark, copy and export block structures.
Block selection preview:
Block placement preview:
Structure export and load interface:
Marker usage: click on a start block, then click on end block to store the structure. Then you can place it in the world.
Pressing "N" will rotate it counter-clockwise.
Shift + right clicking on anything will reset the marker.
Pressing "Alt + N" will open the export interface; to export the structure, write its name in the text field and press the button.
Exported structures are placed into "minecraft/structures" folder.
To load a structure onto the marker, select it and press the button.
Structure files can be shared with others.
The marker is in the "Tools" tab.