You have a much simpler option -- just buy a large seatbag like a Carradice Barley or Pendle. I have commuted 22 miles round trip for nearly four years using a Barley to carry all of my gear. I commuted on a racing frame for more than two years using the Barley. Two years ago, I bought a touring frame but continue to use the Barley. I briefly installed a rear rack and considered getting panniers, but decided after a short while that the Carradice bag was all I needed to carry my gear and I removed the rack.