Thanks Millet
I'm going with Polyhives board solution as it's ludicrously straightforward.
For your entertainment,
https://ibb.co/rdGQ8Yx
should hopefully be a link to an image of the "super"
It's 170mm tall and the same width externally as a national.The distance between the metal rails is 21"
The deeper box with the bees is 265mm deep and I'm told has been built back inwards to take national deep frames
The solid floor has an upstand of another 75mm
They both have recessed rims for wooden framed Qe/cb
There is a conventional style flat roof