size of the mantis is not the issue here.
ricks idea seems sound. i have another method which is really hard to explain and sounds way more trouble than its worth, but once youve got the technique down its easy. i simply get a net of some kind or a transparent bag (both work) and put the mouth of the bag to the lid of the fly container. i open the container slightly, and have a lamp shining towards the tip of the bag. as soon as i open to container, flies fly out towards the light, to the very tip of the bag. then i just close the container, close the bag, and repeat, but the other way round (have the light shining into the mantis container, so the flies will fly out of the bag and into the container).
like i said, it sounds like more trouble than its worth. basically, use the idea that flies pretty much always go towards bright lights. remembering this helps to keep them under control a bit more, you can kind of direct them where you want them to go.