I use a Plastic Welder. They can be bought off Ebay also. Here is one that I use. It will weld up anything. Just read the book and watch the video and practice on a piece of other plastic, it is easy to pickup the knack for doing it right. On big Cracks, be sure to use some Aluminum Window Screen and just follow the directions.
this is what i did. i used some stainless wire i had. got a plastic weld kit from harbor frieght and a solder gun. do all of this on the inside.
1. bend the wire to size
2. put it in gun, hold to plastic and heat it to proper depth.
3. cut ends off
4. use weld iron and smooth out over staples.
5. smooth out outside.
make sure to practice first.
i got this idea from a product that cornwell tools have.
It's not to old. Maybe 5yr. I'll have to look at that spot tonight and see what it looks like. I had some spots that where offset. I just bent some small peaces. You should be able to make some to go in a corner. I'll let you know.