I don't feed with crickets often, mostly just as a bonus supplement for the larger females that eat a lot. However, I keep them in the cricket keeper and use the tubes to dump a single cricket into a small deli cup. From there, while holding the lid most of the way on the cup so the cricket can't jump out, I remove or maim the jumping legs so the cricket can't go very far if it does get loose. It's pretty easy to grab with tweezers then. The trick for me was just to reduce the space it can escape to, and that meant shaking them out of the tube one at a time. I only ever need at most 2-3 at a time though and I'm sure this would get tedious if I needed more. As a side note, I transfer them into the deli cups outside on the deck, just in case they escape during the shaking.
Another option if they don't have working jumping legs anymore is to just hold the deli cup open in front of the mantis so they can grab the cricket straight out of it while it tries to climb the walls of the cup but can't.