After printing about 7 failed attempts, I got tired of wasting plastic and remembered I had a 3D scan laying around. This helped A LOT!
The model now fits and I'm finishing it off with a power button mount and some brackets for the LCD. Fingers crossed this whole contraption is strong enough to support the weight of my LCD.
In the meantime I've also been working on the IO shield. Had the same problem, this iMacs curves are just too pretty to be modeled by a novice like me. Made a scan here too and now it fits perfectly. (no really, almost better than the original)
I'll check back in when the model is finished and I can start glueing this thing together.