Well u can basically construct a new mat for your trunk using a 1/4" - 1/2 compressed wood board. Cut out the pattern using the existing mat. After doing so, buy some carpet, the same one for the trunk lining. What I did for a friend was the same thing but instead i put 2 hinges somewhere around the middle so that he could access the wheel well.

If you don't like that idea, just basically use support. Try to make it more of a # structure. Make sure it's long enough where it rests on the frame of the wheel well. G' luck!