whit ye babblin aboot, badruck?
Yes, it would be nice if we could all just click our fingers and summon a correctly sized tervigon/tyrranofex into being, but we cant. We must make do with what's available. to whit: carnifexes. Yes, its a bit small, but the only bigger models available are:
Trygons - totally and utterly the wrong shape. There is a clear description of a tyrranofex in the nid dex, and it says "heavy armoured body supported on six legs". This is practically the opposite of a trygon.
Forge World scythed heirodules: definitely bigger. 10 inches long, in fact. Perhaps a bit
too big. It also lacks for the right guns/legs and stuff. That means if you want to take this route for a tyrranofex you must first buy a 90 GBP resin model then take a hacksaw to it, then spend a good deal of time sculpting a pile of guns and proper legs onto it. Most people instinctively think "gnnghgnghgnghno" to that idea. Personally, I find the it quite appealing, but then, i find the idea of a owning a squadron of Nightwings quite appealing, too. Doesnt mean Im actually gonna do it, though.
So, having ascertained that the only model available upon which to base our tervigon/tyrranofex is a carnifex, it comes down to: how would it look?
Well, the chapterhouse tervigon certainly looks like the picture in the nid codex, i think. As to how youd fit 3d6 termagants in there, you wouldnt. They get squirted out as wee eggy things and grow to their full size outside of the tervigon. Its not a bloody mammal, for goodness sake.
There are several gamers in our group who could sculpt a better model than the chapterhouse one, but its all a question of "is it worth it?" If you want to spend months sculpting your own tervigon, then go for it, but for those who dont there is now a cheap and easy solution. Thats why i suggested it.
Personally, I reckon a tervigon can be based around a carnifex, but with some fairly major posture changes. It certainly shouldnt be bipedal. all six legs on the ground (possibly centauroid). big fat abdomen like a spider or similar. Keep the 'fex head and upper torso, and try to use the armour plates on the top of the abdomen. I'm not familiar with the 'fex sprue, but I reckon it could be done.