Hi Phill,
the reduced wooden lathe above the floor material
At the moment it is 22mm? If the entrance were only a bee space wide, that is all the cover would be around the rest of the floor (between floor mesh and brood box). 8 mm thick (or thin!) might not be robust enough.
It all looks very good. OK, the next one(s) will be easy! The first is always that much more difficult.
You now need to make at least two more! I always advocate two colonies minimum, if possible, to make the beekeeping that much easier, and then you will need more kit, for swarm control, eventually.
Don't believe anyone who says they keep bees with just the one hive only. Just not true (unless they have a Dartington or similar), unless they take drastic measures to prevent swarms, use shoe boxes to hive swarms, or just let them loose (neither efficient nor (possibly) socially acceptable these days! Spring cleaning is easier too.
Regards, RAB