The shape of the Monte channels might be different than the GP channels. Maybe the roof isn't as rounded.
I would do this:
Put Gorilla Glue on the ends. Put a lot of glue on, but not too much though, so it doesn't squish and push out, leaving a mess on your paint. Put a rag over the rail ends and roof so they don't scratch. Put 1 cinder block on top of each end, or 2, or 3 cinder blocks. Let the glue cure for 24hrs. with the weight of the blocks pushing down. I bet that will work. Doing it in a warm garage would help. Heat/warmth definitely makes plastic more malleable.