I've decided to try building a heat exchanger core with the simple cross flow tube design. Below is a link to CAD model of the concept. I'm going to use polypropylene drinking straws for the tubes. They have a wall thickness of only .0055 inches (0.14mm). I'm still not sure how closely I will space the straws on my first attempt, but either way, it will be an experiment. I think I'm going to use this fan, which I found on Amazon
Coolerguys 120x25mm Blower Fan 12v with 3pin Connector. It delivers 17 CFM and the specs say that it's very quiet (<20 dB)
Flickr: Drinking Straw HX