Same for me (as far as "out of square" square tubing).
Hopefully not too late for you... I'm a terrible carpenter so this was a learning experience.
We screwed wood furring strips to the square tubing using self tapping flathead screws lengthwise, and crosswise at ~ 4 foot intervals. Then I bought an air stapler using narrow crown staples. This was the best purchase ever as it was fast on the ceiling work, and made it easy to re-attach the trim to the wall panels after I insulated the walls.
Once I had the strips installed, I worked to get the ceiling panels somewhat square to the walls, using luan strips to finish the crooked seams left by getting the ceiling square. Then I just stapled all the ceiling panels to the wood strips, adjusing the staple depth to what looked best without pulling through the luan. I previously had stuffed 1" foam insulation cut to fit between the square tubing throughout the roof. The goal was to reduce metal fastener contact to the "cold side", cutting back on condensation. Pretty good results so far.
To bridge the gap between wall and ceiling I built flat panels with trim the same width trim pieces as the wall/ceiling and stapled it to wall and ceiling after insulating the curves with fiberglass. (yes,lazy) This covered the "crookedness" between the new ceiling edges and the wall to dress it up. I had all kinds of ideas to make curved panels, build angular anchors and such, but once I made the panels, it was a cinch to staple them directly to the wall and ceiling (through luan and into the furring strips at most locations)
I realize I don't have pictures in my gallery of the process, but you can see the finished product, and this build is where my ideas for the ceiling came from:
http://www.tnttt.com/viewtopic.php?t=40504Here's a couple of pics of mine finished.


pic of fan install, ceiling detail

view of overall ceiling detail