The one I'm planning is a bit smaller than TK's, but similar in design.
I'm planning on using a GW Necron Tomb Spyder for most of the work. (Blasphemy I know but desparate times call for desparate measures)
The carapace, legs, and claws are exactly what I envisioned on the Cancer MMG. My conversion plan is fairly simple:
1. Stretch the legs out so that the piece will stand on the legs instead of on the flying base it came with.
2. Assemble the Tomb Spyder, minus the head bit that comes with it.
3. Dremel out a small opening in the top of the carapce for the torso. (Thinking of using either a PV torso, Chasseur, or maybe even Big Bob Watts upper torso, as he comes sans arms.
4. Affix the torso into the opening, green putty where necessary, and find a pair of suitable weapons for the MMG's.
5. The claws already have spots under the carapace, so no conversion needed there.
Presto. Cancer MMG with only a minimum of conversion work.
Thats my plan, at least.
SP