I've been working on an idea for an air2air exchanger as well. Even being able to get fresh air half-way from outside to inside temperatures would make hot and cold season camping more comfortable. Commercially available exchangers are EXPENSIVE and too big for our application. I've been trying to come up with one that would be small, easy to produce, and work well. I was planning on using 12v. computer fans to produce the airflow. A prototype has NOT been built yet.
My idea is to use hardwood strips on the edges with aluminum foil as the separator between the two airflows. The hardwood strips would alternate like lincoln logs allowing flow from north/south and the next layer allowing flow from east/west. So each layup would be two sticks on the edges running E/W, a layer of foil, two sticks running N/S on the edges, a layer of foil, repeat. The foil would be attached to the strips with double sided tape as a seal. Essentially. I was going to build up a cube of these and the flow that is north/south through the cube would not mix with the flow that is east/west, but they would be able to transfer heat from one layer to the next. The rest of the puzzle would be to create appropriate ducting adapters for the fans and intake/exhaust on each side of the cube.
This is a quick and dirty sketchup drawing of my idea. You will need to orbit around to see how the flows are kept separate. The corners could have a bolt through them to hold the stack together, but I think that the double sided tape may be adequate.
Let me know what you think.