I found a bunch of BST cats on my fennel. One is 6th instar (he's H.U.G.E., so I decided he needed his own designation...haha) the others are 3rd instars. Once they make their chrysalis, I was going to cut a window* in a 16oz recyclable foam cup, cover that with a piece of firmly taped plastic wrap, then cover the top with 2 layers of nylon stocking.

The reason I want to use foam is because it's 'grippy' for them to climb on once they eclose. (some will not be attached to sticks...they like the screening on my cage and a lot tend to attach there, so I will gently remove them for relocation.)

*the window is so I will be able to see if parasitic wasps come out from a safe distance. I know myself, and seeing them that way is preferable to looking down into the cup to a nasty surprise, screaming, and flinging the cup across the room.