So the wife suggested I show the conversion in steps rather than one big post when it's done, which I think is probably more sensible.
I wanted to make my Tyrannofex fit the description in the codex at least partially, but I didn't want to just convert from a carnifex kit. Whilst looking through my assorted bits boxes, I came across a bunch of old nids that I still though looked good, namely the "Alien Queen" style hive tyrant and some parts of the old carnifex. I decided that these models would make up the bulk of the conversion. The description in the codex goes: "Clad in a dozen layers of ablative chitinous armour and supported by three pairs of massive legs", and then "...its body houses several breeding chambers." These were the main draw of inspiration for my conversion.
1. I took 2 old carnifex carapaces and stuck them together. I filled them with hot-glue from a glue gun. It dries rock solid and can be carved after it's dry. I ran out of green stuff so filled smaller gaps with 8 year old milliput, which never really dried... mind you it did look a bit foosty.
2. I pinned the underside of the carnifex chest to the abdomen section using really solid wire from a wire coat hanger. I needed to use pretty hefty pliers to cut it though, but it's great for large heavy models like this.
3. I pinned the body of the old hive tyrant to the lower chest section of the fex. I used more hot glue as a filler.
4. I glued on 2 spare plastic carapace sections from the newest carnifex kit. The one of the front has spines on it, representing one of the three weapons the tyrannofex has: the stinger salvo.
This made up the bulk of the structural work, and left me with a solid base to work on. I'll be doing on some sculpting next.