What is an effective method for putting on blue rubber butyl stoppers?
From what I can tell from this machine (https://www.youtube.com/watch?v=TwRtsAG5Oy4), the blue caps are put on the bottles first, then a loose version of the aluminium neck seals are put around the top of the capsule, which is then pressed in the machine to seal the lid hence if you use many gas sampling techniques, it might worth investing in purchasing a similar machine/technology to seal your samples, which would be the most efficient in the long run.

EDIT: as pointed by @user137 there are also hand crimping tools, which you can use, which are considerably cheaper, which as @user137 pointed out, can be found here