Green & Blue Bottle Fly Pupae.

How I hatch.

Transfer into a vented container. A cricket tub works well for me. But anything with good ventilation will do the trick.
Then you can either add a few casters to mantis enclosure and let them hatch in there.
Or leave all or some out in a warm area 20 - 25c is best to start hatching. As soon as they start hatching, put in the fridge to slow the hatching down, and it also makes them slow and easy to handle.
Cold slows the metabolism and metamorphosis down, and heat speeds it up.