I slipped on the stairs and injured my tailbone three days ago, and have been unable to sit without pain since then. Your website has been very helpful. I quickly realized that this injury may take some time to heal. In the meantime, my job requires hours of sitting, and driving is very difficult. Internet searches turned up lots of pillow options, but I wanted relief NOW, so I bought a pillow at a local medical supply store. It didn't work, possibly because I am quite overweight, and the pillow just flattened out. One website recommended an extra firm pillow for heavier people. Since I have to drive and sit every day, I didn't want to wait for a mail order. They say necessity is the mother of invention, so..... I started experimenting, and this is what I came up with:
Using inexpensive children's foam swimming noodles, fashion a U-shaped cushion which fits you, your chair, and your car seat. Use a serrated bread knife to cut the noodles, it works very well. If the simple U works for you, use duct tape to hold the U shape. Wrap the tape around one end a couple times, then bring it across the bottom, then wrap around the other end. Put a piece of tape on top of the sticky part which is holding the U ends together. Another option you might try instead of taping to hold the U shape together -- diagonally slice the ends of the noodle to make them wedge-shaped. Then slide these wedged ends into the back of your car seat or office chair. This held very well in my car.
If you find that you would like the cushion to give you a little more of a forward tilt or lift (which seems to alleviate the pain better), cut another piece of noodle and place it straight across under the ends of the U shape. Use tape to wind around and attach at the ends. Then you may also use the bread knife to cut a curved indentation in the center of the straight end of the noodle if your tailbone is hitting this "cross-bar" noodle.
This is an inexpensive way to create custom cushions which fit your body and your needs. You might experiment and make different designs for different places (car, office, etc).