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Make sure your gazebo frame is truely squareby measuring diagonally from corner to corner.Adjust until you have equal measurements
Check all measurements of the corner hips again for square.Also make sure the roof panels are the same size, also they may overlap in the center,as this is hidden by the seem cap.
I went and checked and mine overlap on the center beams.
I don't have any advice....Perhaps overlapping them? Curious though, you posted last week that the dealer wanted $500 to assemble and you opted for the DIY. (of which, many times I am a proppent of., ) if after all said and done was a fair price for the assembly?