In article ,
"Christian McArdle" writes:
IIRC, grommet strip - but you'd need to go to somewhere like RS for this.

TLC do it. Useful for rectangular or unusual shaped cutouts.

The sheath stripped from T&E by running a knife down the
side of the earth conductor (like what you're not supposed
to do;-) makes excellent grommet strip, and is guaranteed not
to react with the cable sheath of course. On straight edges,
it needs glueing on, but so does real grommet strip.
Different sized T&E gives grommet strip suitable for
different thickness metalwork.

Another form of grommet strip is to take a grommet for a
larger hole and cut a piece out. I usually end up doing
this for the smaller holes in architrave back boxes.
The trick is to not cut out quite enough, so the grommet
is held in place with some compression force. You could
glue the ends if you wanted, but I've never found it
necessary.

--
Andrew Gabriel