It is true my box doesn't meet those minimum size requirements but the ONT is the only thing that needs to go in this box. 350x700 seems like a very large box for just the ONT. Although I have never actually seen one so maybe it's bigger than I thought. The box I made I drilled some 20mm holes in the sides for ventilation. I was planning on making a glass panel in the front to see any lights that may be interesting.
My RF patching is going to be in the ceiling (at point P), the network patching (48 port patch panel + 24 port managed switch) is in a cupboard in the middle of the house (P). The alarm panel is located in another cupboard.
I have included the outline to show why there is no point patching from where the fibre (F) comes in. The living areas are at the bottom of the image, bedrooms in the middle, garage at the top.
I think I have to give up and just install the ridiculous sized box (and thus remove insulation from the south facing wall) otherwise there is no way they will install. Wife is concerned they'll stop coming and we'll never get internet.