I was thinking about using this as the base unit:
PAX Wardrobe frame - birch effect, 100x58x201 cm - IKEA
pax-wardrobe-frame__43277_PE139000_S4.jpg

Then adding some shelves as the viv tops and bottoms.
Maybe with an internal set of drawers to keep stuff in at the bottom (who wants to be bending down to a viv at floor level?
komplement-interior-chest-of-drawers__0134759_PE291373_S4.JPG

I was thinking about 3 vivs in one of these wardrobe shells. Then, for the vivs we already have, I just need some shelves to sit the 4 ExoTerra vivs on. The other alternative is that the ExoTerras will fit on shelves in the wardrobe shell, so rather than building the vivs in, I just slot the Exos onto shelves (more expensive, but when you already have 3 60x45x45 Exos, and a 90x45x45 Exo...).