On Wednesday, 29 April 2020 11:32:00 UTC+1, wrote:

Aren't there any knock-outs to, errr, knock-out?
If not, you could get a nibbler into a 4" gap and then improvise grommet
strip from anything suitable (saw blade protector, slit sleeving, T&E
outer ...) and then use high-temp silicone to form a seal.

Is that 18th edition passable?