I found my alternative in the HITECH Miniatures Hellfire Cannon. It is a four-barreled cannon that is the appropriate scale and it was available at a good price, so I picked one up. The cannon itself comes in four pieces.
The base comes on two large runners, which were a bit difficult to cut off without damaging the model. They left flat circles on the bottom of the treads where they were removed, but you shouldn't be able to see these.
The gun looks pretty good, though it has a broad area on the back where it seems to have been sawed off from a larger block of resin. This had to be shaved down to make room for the final piece.
This is a vent that covers the back of the gun, where the rough area is shaved down.
The gun is made from a plastic-like resin, similar to Privateer Press models. It has minimal flash, but it has some bubbles, similar to what you'd see in Finecast. Fortunately, most of the bubbles are on the underside so they won't be visible on the table. As you can see from the photos, the bubbles on the upper surfaces are generally small, and you could paint it up without repairing them if you wanted.
The Cannon is easy and quick to assemble, unlike the Thunderfire Cannon. Here it is.
I really like the model and I think it will fit well in my army. My only disappointment is that it lacks and broad spaces to put freehand on. I will be going over it and filling in the bubbles with green stuff.
As an aside, it comes with this fellow:
He's a nice model and is well cast. He's also on an appropriate base for 40K. I won't be using him, but he can easily stand in for a Mechanicus Adept. Speaking of that, here's my Techmarine who will be running this Thunderfire.
He's an old Techmarine, so he doesn't have a full Servo-Harness. In order to make him WYSIWYG, I crammed all of the parts of a servo harness onto his left arm. It certainly looks overdone and silly, but I kind of like it. It looks like how the Imperium would do things, and it matches the weapon arms of the Centurions.