I have this same problem. I bought #443 furniture factory. After joining the wall sections together with a polystyrene joiner plate and a piece of H stock to stiffen it, I was ready to join the sides to the ends. I did a temporary join with masking tape and found that the walls were 1/8" wider than the roof panel, so it just falls through. Investigation found that both end panels were warped from top to bottom causing them to spread at the top (or the bottom). I tried heating the panels but it only helped a little bit. Unfortunately all of the panels have been prepainted and lights added. After removing the lights it is very likely that dropping them in hot water will damage the paint - so I wasted 3 weeks. Further, I assumed that the stops on the back of the wall panels would line up the side panels with the end panel - they don't. The stops keep the side panel about 1/32" too far out. I will have to cut them back so that the sides fit correctly.
This was a lot of work, so I am disappointed that upon reaching the final assembly stage the parts do not fit correctly and I have to return to square one. At the same time I bought the #504 dry goods store. It is a lot smaller and, based on what I know now, I will test fit everything using hot glue before doing any kind of decorating.
