I do something similar with my flies, but hatch them in a thin plastic half pint cup, with cheesecloth over the top and a stopper low on the side. Flies hatch out (sometimes) and i just paste some honey water on the cheese cloth. then for feeding time, take out the stoppers on the cup'o'flies and the mantis cup, push the holes together and gently tap, the flies just go in, no escapees.
Only issue i'm having is getting the flies to hatch, I bought a half pint of castor maggots, store them in the fridge and I'm finding the hatch rate to be very very low, more so as time goes on. Anyone got any tips to make my supplies last longer?