Greetings
        My copy of the ABC XYZ of Beekeeping, published in 1978, has complete plans for the OAC pollen trap. Their original version is very simple and inexpensive to make.     Most of the modifications that have been added over the years are unnecessary or ill advised. Drone escapes are essential, screen on the bottom tray is not.
        I had over 300 of these installed in the 1980s when I sold bee pollen for human consumption. My wife and I produced about 4000 lbs per year. Trapping pollen is labor intensive but if you have a market for it, you can make more money than just about any other form of beekeeping. I will gladly answer specific questions about trapping, storing and selling pollen.
        I no longer own any bees and sell no products, being solely involved with bee research at the University.

Peter Borst
Ithaca NY  USA