Wow, you've really put some thought into it! I'm with rabernet, I dump the flies into a plastic cup with the dust, shake it up good, and dump the flies in the tank; usually some dust ends up in the viv. But I dunno if I'm up for making such a fancy apparatus just to avoid it I use the Repashy ICB supplement, which has a finer grind than Herptivite, so that helps a bit (sticks to the flies more).