Well, first off, what do you mean by "scratch build"? Are you planning on using plasticard and GS to make something, or are you going to kitbash and make a super conversion? Also, by "two animals" I'm assuming you mean either the Thundertusk or the Stonehorn, is that correct?
For the Ironblaster... The biggest things you're going to need to represent are the cannon itself, as well as the three other statlines (the rhinox, the leadbelcher, and the gnoblar scrapper). So, you
technically need to have at least three models with the cannon to showcase everything that must be brought to the table. Granted, there are a couple of cool conversions that you can find throughout the Gnoblogs (although I lack the links to portray them, sorry!).
However, for the big beasties, it'll be tricky to make one device that can cover both, because they both have very different roles on the field. But if you want to build an "engine" that can represent either of them, that is to say, a machine that can be used as a substitute model for them, here's a fun idea:
1. Make the machine large, about double the avg. Ogre's height. You can make it look either like a Trojan Horse (or Rhinox

), or like a smashed together double-Steam-tank mess, or what have you.
2. On the top, towards the rear of the rig, place a large, tear-drop shaped engine that has a huge blast coming out of it. I'm picturing Space Shuttle flames and stuff.
3. Make the engine so that it can be rotated; So, for representing the Stonehorn, have the engine pointing backwards like it's pushing the machine forward at a ridiculous speed, and for representing the Thundertusk, have the engine pointing upwards, as if it's shooting out a freezing sphere.
4. Put lots of sharp looking things on the front of the machine, and some "arrow slots" where chaintraps and harpoon launchers can be fired through.
Wow, that actually sounds like a fun conversion. Too bad I lack the money atm for such a crazy device

Here's my
professional artistic idea of such a random device.

I know, I know, don't applaud me too much

lol... j/k. But, in all honesty, I hope this helps give you an idea for the two big guys.
Oh, and for the Ironblaster, it'd be awesome to make a giant mock-Steamtank, using the cannon from the IBlaster kit as the proportion size. I'd love to see the face of an Empire general who'd see that
