I used Plastic weld, and glued them shut. Sometimes I use painter's Caulk if the gaps are tight enough, then prime and paint. I might also (very carefully) use a heat gun to get the curves to match up. But for the most part I just use a pencil, marking where I need to sand it down, following the curve of the lid. That alone is the best way, just leave yourself enough to play with. Can't go back, once it's gone, it's gone.