How to move a wild Phormictopus cancerides from danger

[Vanessa]

(I sense things may not be so simple...)

It could go smoothly... or not. That is a species who is fairly skittish and fast. They could just hunker down inside the box, but they could run right out again. It is better to use something that confines them closely. I would choose something that would allow you to see them at all times so you don't get greeted with a defensive, fanged, Jack-In-The-Box when you go to release them.
I would opt for a clear plastic food container which is wide enough for about an 8" leg span.