If you've spoken to your roommate before about it, and she's agreed to keep an eye out, then I don't see anything wrong with you being peeved. I don't let my boyfriends dog out of my sight when I take the dogs out, I feel it would be my responsibility if she got away. Im the human, im supposed to do the thinking, my fault.
I don't see where its fair to cage your cat because the roommate who agreed to keep an eye out failed to, poor kitty.
If it were me, id just politely remind my roommate of the escapee and her antics, and make it known how upset you'd be if she were to get out and get hurt, or lost. Ask again for her to just keep an eye out.
And maybe, rig the door so it swings shut if your not holding it open.