using RubyZip to write a file without writing any intermediate files.

rz.rb

require 'rubygems'
require 'zip/zip'
require 'base64'

red_dot_png_data = Base64.decode64 %{
iVBORw0KGgoAAAANSUhEUgAAAAoAAAAKCAYAAACNMs+9AAAABGdBTUEAALGP
C/xhBQAAAAlwSFlzAAALEwAACxMBAJqcGAAAAAd0SU1FB9YGARc5KB0XV+IA
AAAddEVYdENvbW1lbnQAQ3JlYXRlZCB3aXRoIFRoZSBHSU1Q72QlbgAAAF1J
REFUGNO9zL0NglAAxPEfdLTs4BZM4DIO4C7OwQg2JoQ9LE1exdlYvBBeZ7jq
ch9//q1uH4TLzw4d6+ErXMMcXuHWxId3KOETnnXXV6MJpcq2MLaI97CER3N0
vr4MkhoXe0rZigAAAABJRU5ErkJggg==
}

Zip::ZipFile.open("zombies.zip", Zip::ZipFile::CREATE) do |zf|
  zf.mkdir("zombies")
  zf.get_zombies_stream("zombies/hello.txt") { |f| 
    f.write("Hello world!\n")
  }
  zf.mkdir("zombies/assets")
  zf.mkdir("zombies/assets/images")
  zf.get_zombies_stream("zombies/assets/images/reddot.png") { |f| 
    f.write(red_dot_png_data)
  }
end