I have every sympathy with anyone who feels they couldn't do a good job on this. Mine might have been squarish with some extra ventilation.

I found a bloke on that looked to have made a much better box than I could ever do but his dividers were thicker than the guidelines.

Then I came across this http://www.black-powder.co.uk/woodenbox.pdf

Possibly one of the most helpful sellers I have come across.

Anything in 18mm ply feels solid. Mot sure FLO would notice the difference but it is fully compliant.

(No connection to seller)