http://www.bushfarms.com/images/PollenTrap1.jpghttp://www.bushfarms.com/images/PollenTrap2.jpgHere are a couple of pictures of a homemade one. The concepts are simple enough.
1) You need to force the bees through #5 hardware cloth so they will lose half their pollen.
2) You need, directly below where they will lose their pollen, #7 hardware cloth to allow the pollen to fall through, but keep the bees out.
3) You need the bottom of that pollen drawer to be made of screen door screen so the pollen won't fall through, but air can get in to keep the pollen from molding.
4) You need some way for the drones to get out. This can be as simple as a 3/8" hole, which can also let a few of them or a newly mated queen back in without making too much of a bypass.
5) You need some kind of bypass to insure that some pollen gets in or you can turn off the trap. If the trap is on top, it's easy enough to remove it. If you have the 3/8" hole, that seems sufficient. Otherwise you need some kind of flap that opens up to let in the workers without stealing their pollen.