If you have the holes to mount it, and maybe even if you don't, a normal rack is definitely the way to go. Performance bike has theirs on sale for $20 if you can wait for it to be shipped to you.

http://www.performancebike.com/shop/...tegory_ID=2325

I've got to say, if you can ride 50 miles, even without a backpack, and not have a big old sweat mark, then you're in much better shape than I am. When I ride 50 miles, I don't worry about the sweat. Still, I wouldn't want to do it with a backpack.