As long as the lid holds up as you say, it looks like a good plan.

I do the same empty cup approach as Bill and Carlos. You have to constantly tap any culture with live flies in it to keep them from climbing out. My biggest concern for this approach is the flies that get up to the edge of the cup when I'm putting the lid back on. A few escapees isn't a huge concern, but the little guys that get crushed in the lip of the lid is. Having crushed flies stuck to the lid is bad for moral for the rest of the culture.