Cannot get the K8400 body panels to line up

I’m following the step-by-step online instructions for the K8400, and I’m most of the way through the second page of assembly instructions. They stress, on a number of occasions, that when attaching the corner pieces, there should not be gaps between the panels you’re joining. I’m really struggling to meet this requirement, especially now that the large and medium corner pieces are attached and I’m on to the small corner pieces.

I’ve been clamping the panels together, and I’m tightening the bolts on the corner pieces to the highest tolerances the plastic can bear, and I still have gaps, varying from a hair up to about a millimeter. I swear I’ve done the best I can, and I’m at wit’s end. Can someone tell me: How important is it to completely eliminate gaps? I need to know if I need to tear this whole thing apart and start over, or if I can still save it.