if I understand correctly , it's the same set-up as a D.E. Hive or the D E mod kit, they protect that space every day when open.
Sort of similar to the DE mod kit, except David Eyre does not like screened bottom boards, so the mod kit is a solid bottom. But yes the entrance size on the mod kit is ~ 18-3/8".
I believe the DE hive has the same dimensions as a British standard hive, so the entrance is a tad smaller. Perhaps someone with a DE hive can let us know.