For what it's worth, here's a little trick that I used when I redid my carpet on my hatch covers and what I did to do the corners:
Step 1:
After the carpet is glued to the top of the lid your ready to cut those corners. Fold the carpet up, one side at a time and mark a straight line along the corner and the side of the lid. When your done one corner should look like the pics below.
Step 2:
Now you can cut your marks. The first time you do this I recommend that you just cut the straight lines first so your piece that is removed looks like a square. Then you can the angled pieces for where it rolls down the inside of the lid. This is what you should have when all your cuts are made.
Step 3:
Ok that's pretty much it. Just fold the corners over to make sure they fit properly. If you have excess carpet rolled over into the lid area that's ok you can cut it after the glue dries.
Pretty simple...
Laters....