You can 3d print one actually, it works pretty well. The only problem is the alumina has a higher volumetric heat capacity, of about 3.5, whereas PLA has 2.25. Which is pretty high. The thermal conductivity isn't a big deal, the main impediment to heat flow is in the air channel, not the solid material, plastic or ceramic. I commented elsewhere in this thread on how to print your own exchanger relatively easily. You can also get your own fans, so this begs the question of why you would pay $600 for them! Also you need a pair of them, you have to have two of those modules operating, one blowing air in and the other out at any given moment. Then they reverse direction. 70 seconds is a bit long for reversal time, it's better to make it shorter if you can. If you are diying one you can do that. If you try to mod an aliexpress one it's only going to cost more and probably not be as good to be honest, because you can't optimize it. Not hackable!