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The couplings I used are
which fit in a 12 mm hole, at least 7 mm deep. (I missed the last part on my first try, making the holes only 5 mm deep, resulting in the tubing sticking since you couldn't unlock the connector because it struck the back end of the hole.)

E3D also sells
wich has the aforementioned 1/8" British Standard Pipe thread.

Both of these are drilled through, and are really easy to work with, which suits this usage perfectly.

You can probably get the same type of connectors elsewhere. I just happened to add a bunch when I was ordering a new hot end from them anyway.