Since I already have a bike rack on the front of my TV I wanted to find another way to carry gear on the bike rack of the Columbus (25" X 70.5"). I found a nice, waterproof bag on Amazon that fits perfect (with enough room to store a 25 quart cooler beside it). It already has the tiedown straps included and the quality seems to be above average.
Of course I haven't tried it out yet but thought it may be an option for some of you with the same rack.
Bag Link (24 X 59):
Cooler Link (20 X 12):