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I used to work for scientific instruments company, and we would seal sodium iodide scintillators in open ended metal housings using epoxy resins. Air bubbles were a problem, but we would get rid of them by first putting the pourable resin in a vacuum. The bubbles would come right out.
I don't have a hot air gun, so I fold a layer of aluminum foil over top and bottom of the heat shrink tubing before using a small flame. No soot, and seems to work better.