For Warmahordes I play both Mercs and Crocks. Unfortunately they do not make a gargantuan for the crocks. I stumbled upon this sarcosuchus on Shapeways.
The measurements looked right so I ordered one. The crock turned out to be just the right size to fit into a cut out 120mm base. When I received it the whole thing was hollow, so I back filled the model with plaster. Then I added the rope supports for the howdah and bulked up the sides so it looked like the ropes were taught. Here are a couple of pics with Bloody Barnabus for scale.
I wanted the howdah and mortar to look like they were scavenged. I constructed the howdah to look like the ripped out wall from a barn or deck of a ship. Then I constructed the mortar from plastic tubing and green stuff. I used a hot pin head to distress the surface and chipped the barrel. I tied it down to the surface of the howdah and added a green stuff sandbag to support the back.
Here it is in place.
So far I was lucky enough to find a used Bloody Barnabus to convert into the driver. Lots of cutting and now some re-building. I have a witch doctor on order to add in as a gunner.