a friend of mine has been thinking of putting one together for his skaven army.
Some ideas for what you could try: a worm-like thing, with a stitched together look, lots of spare limbs/heads/tails from a skaven boxed set poking out from it, large Jaws made of something, maybe even make them the mechanical part. Or, you could give it a form like a flesh Golem
, give it limbs (any number), have it walk along on 2 or 4, no head, just a big maw in the middle of it's torso. Or take inspiration from the thing ( see 1 2
For the doom flayer (that's the new weapon team option right?), I thought of a couple of ideas. Either have a couple of clan rats carrying a giant buzzsaw, one on each side. Have them each holding onto the haft with the engine in it, shoving the whiring blade towards the enemy. Or have them pushing a spiked/flailing roller in front of them, like an old fashioned lawn mower (the "man mower" I like to call it).
Anyway, just throwing around ideas, can't wait till I see what you come up with!