I was thinking of an open bell. I don't recall if butane is lighter or heavier than air. That determines whether the bell points up or down.
edit:
I was thinking an open bell for safety's sake; but I now think atmospheric pressure wouldn't be a good test.
So.... maybe a 3/4" clear acrylic tube. threaded on one end for the spark plug with a pop-off valve on the other end?
Maybe I'm watching too much
Integza.