Rotate and center text with Prawn as a watermark, e.g. for "[PAID]" on an invoice.

prawn_stamp.rb

create_stamp("stamp") do
  fill_color "cc0000"
  text_box "[PAID]",
    :size   => 2.cm,
    :width  => bounds.width,
    :height => bounds.height,
    :align  => :center,
    :valign => :center,
    :at     => [0, bounds.height],
    :rotate => 45,
    :rotate_around => :center
end
stamp("stamp")