So I have been wanting something more efficient for my car's trash than I had been using.
I wound up just kind of winging it - I made a couple of squares 13"x13" and used some store-bought binding sewn into the seams for the straps. I also used some 1/4" nylon boning to hold the top open. Here it is with a plastic grocery bag liner.
This is what it replaced:
I may try to lengthen the straps so that the bag can sit a little lower in the car, but I am pleased with how it turned out overall!