If it's only a slight lip you should be able to sand it down. If it's really big you should consider remaking the joint. (Which might require cutting one or two new side panels.) Maybe the thing to do is sand the joint flush, wire the side panels together at the ends and put a spacer in the middle to bow them out to their final shape. Then look at both sides closely to see if they have fair curves and match each other. It's pretty important that the two side panels have the same shape and that their length on the bottom matches the length of the outside edges of the bottom panels. Also, give CLC a call. This probably isn't the first time they've heard of this problem.