There a plastic melters out there made for this sort of thing.
Closest pic I could find. The thing in his right hand
Basically you melt the edges and melt them together.
As for the center support, its hollow. But its easy enough. Cut it out then either use the old support or some other plastic to trace and cut the U shaped hole. Then just melt it in. Youll prob. have to use a filler to even it out then get it painted.