I had to pick up my brother at the airport and then go to a wedding, leaving my car in a mall parking garage for a few hours. With a hatchback there's no hidden trunk space for a suitcase and I'd already gotten broken into leaving a bag out on a seat. There are a lot of homemade car hacks out there on the interwebs so I scrounged around until I could find some ideas I could use.
Step 1: Find the shape of the cover using cardboard. Now all others started with a big piece of it but all I had were produce boxes the cat didn't like anymore, so I cut it up and taped. I wouldn't recommend this method though.
Step 2: Transfer to Wood and Cut Out. I used mdf and a jig saw, cut out the basic shape and then had to put it in, see where it got stuck, cut more wood out - repeat until it fits. The center bowed under so I added some supports.
Step 3: Cover with (cheap) carpet. I bought it for $15 at Walmart and glued it to the top. That was all the time I had before the wedding and it worked perfectly.
Step 4: Paint to hide the gaps. You can see how inaccurate my pattern was and my not so good jig saw abilities. It worked though.
Step 5: Enjoy. I need to paint the edges better but for now it's fine.