One of the things about home construction projects is that there are quite a number of thingummies that one knows the existence of (or, often, doesn't know the existence of, until one discovers them and realizes that they are exactly what one needed), but one doesn't know what they are called.
Sometimes these names are rather ... evocative.
Today I learned that the thingummy that I was looking for is called a "cable gland".
Crossposted from Dreamwidth (original here), with comments. Comment here or there; comments here will eventually be duplicated to there.