You'd probably be better off just making one on your own with paper. It's not hard but when you remove the foam stuff it leaves excess material on the edges and so that makes it hard to tuck back in, and look right.
I don't know how to work on cars though...
3m spray adhesive would be your best bet. I use to make speaker boxes back in the day, and this stuff works great for holding down anything. Just make sure you let it tack up before you lay down the leather.