Yesterday I did some additional performance testing. And according to the calculation tool here is the results. At full fan power (fan specification says 700 m3/h) COP is 59%, at half power - COP 69%. So I happily left it running at half power...
This result satisfy my expectations, I only invested 1/4 of a factory-made HRV price
I think I could improve COP by somehow improving air distribution between each layer. I suspect that majority of air is traveling through middle part of the core. Maybe by adding air filter in the chamber where air enters core...
The cost for aluminium sheets was ~100 USD. From these sheets I got 40-45 layers seen in photos.