In order to provide interchangeability with the rest of the circus, I've come up with yet another ruse - this being quite simply a Warre Box housed within a standard British National Deep box, thusly:
To be pedantic, this will actually be a National feeder shell made from (say) 15mm timber/lumber with outside dimensions 460x460mm (c.18"x18"). Inside it will be a Warre box of 300x300mm (internal), made from the same stuff.
Well - that was the plan - and here's the reality:
Still needs filler and paint, and thin plywood cladding adding to both top and bottom of course, but it should be usable in around 10 days or so ...