Sure I can explain although it's better done with pictures than my poor attempts at writing an explanation.
So here goes.
1st I half fill the round tub with culture medium (please note I did overfill this one as I was trying to do it quickly for these photos)
Then the lid with the hole goes on
That is then put inside the perforated tub (it is a snug fit)
On top of that I put a small ammount of wood wool
Then some fruit flies to start the culture off and the lid (you can't see many flies in this pic, but there are about 50 in there)
Now when it's time to harvest them I use a kind of bucket (it contained candy floss when I got it) which have the sides dusted with calcium +d3 (Not enough to dust the flies but it stops them from being able to climb out)
The flies climb out of the culture container and fall into the bottom of the bucket as they cannot climb the sides (This is the fresh culture I just made, but you get the idea)
I usually put the lid on the bucket so as I can be doing other things while the fruit flies are being collected. A little tap on the side every now and again makes sure they keep active and falling into the bottom of the bucket.
I can then dust them as required and am able to easily tip them into a smaller container (film canister) where I can then sprinkle them into each vivarium.
So there you go, this is the method I use and it is very rare that I get any escape while I am working with them.
It works for me and I find it much easier than trying to shake them out of a cup type setup where you often shake out debris of various kinds along with the fruit flies.