Mike,
The doors need to be super clean to fit right. Not even a little hint of sprue left on as well as the mold lines.
As for fit, they fit like a glove but the secret is "Fast assembly". You need the glue to be still wet on all 5 fins and assemblies. Then you need to press the whole thing together (doors up fit into the 2 peg/hole slots at the top of the door and hull) at once while the glue is still not dry and secure it. I use large rubber bands. to hold it.
If it is warped, your screwed. I'd immediately request another kit from GW.
And I will state after 6 years of miniatures with many manufacturers multi piece metal kits, Thunderfire Cannon is by far the worst model I have ever assembled.