I work with air-sensitive organometallic compounds When I was in graduate school, we used to send off samples for x-ray crystallography in ampules that we would seal using a little apparatus. It was essntially a screw cap with a hole and an o-ring that you would put around the ampoule. That would then screw onto (I think) a threaded glass adapter with a ground-glass joint on the other end that you would put a gas inlet onto. All of this could be done in the glove box, then brought out and put on the Schlenk line. I seem to remember pulling a slight vacuum before sealing it.