There are some options. Forgeworld has a Griffon and a Bombard, which could stand in for a Griffin or Colossus. However, at roughly $71 and $93 respectively, they are both expensive models. I was looking for something more reasonable. Another option is to convert a Basilisk model by cutting down its main gun. It's serviceable, but I was looking for something more unique.
I started looking at other company's models, and finally found a likely candidate in the Dust Tactics IS-5 Heavy Tank.
It certainly looks the part (except for not being open-topped), and it retails for $40 (though it's pretty easy to find for $30 online). Conveniently, it comes with another weapon option, which makes it a good stand in for a Hydra.
Scale-wise, it seems quite a good fit. I don't have any Chimera chassis around, but I know that they are substantially longer and slightly wider than a Rhino chassis. I set the IS-5 up next to a Baal Predator for comparison.
It is longer and a bit wider than the Predator, and would have a lower profile if not for the turret. I don't see any opponent being upset by its size.
So how does the model stack up quality-wise? The detail is pretty good. Not quite up to GW models, but good compared to other companies. The plastic is thinner and more flexible than GW's polystyrene, but still feels solid enough. I don't expect it to break.
One odd thing about Dust Tactics models is that they come pre-assembled, with decals already on them. I can see the appeal because it means you can play with them as soon as you buy them, but it feels to me more like buying a toy than a model. A functionally meaningless distinction, but it still felt weird to me.
A very nice thing about the design of this tank is that it is supposed to quickly and easily switch guns. There's a robust tab on the gun that fits into a slot on the turret, and a hinged armor plate that secures the gun from the top. This eliminates need for any conversion or magnetization, as is necessary on GW models. It's a very appealing design feature.
I will have to do some conversion work on the tank to add the hull mounted heavy bolter/heavy flamer, but I think it should be pretty simple. After that, it'll be on to the painting table.