I'm no expert on mics. I've noticed that cardioid mics have the vent holes and omni mics don't. Someone with an understanding on all of this can explain why this needs to be the case. I've had great luck recording with the Croakies with ST-11's. The material allows the sound to pass through just fine. Sticking them in a metal or plastic tube, however, probably wouldn't work so well. Of course, you can try it and let us know what it sounds like.