Here the concept without special plug except mirror!

Speed will be made by Selection (spacebar) Push selection by "P"
The "X Push" is the normal push without special plugin : here it's very easy because you have not a lot of rectangles
so made in a three seconds!


Mirror plug (Right click on a selection) http://www.crai.archi.fr/RubyLibraryDepot/Ruby/EM/Mirror_31.rb




Ps the normal symmetry of a selection is : select anything
take the "Scale Tool"
Take a "cube green handle" make "any move" (of any distance) with it : release it
but now Enter "-1" = you have a symmetry ;)