I made myself one and it's super, well it does the job. I used some fleece fabric and made an oblong, double layered finished about 5" by 12". With wrong sides out I sewed around the outside leaving a small section to turn it the right way out. I then sewed Velcro on each end of the 12" ends and wrapped and closed it around my seat belt, works a treat. Tip: the fabric needs to be something soft, grippy and thick to do the job, so fleece was ideal, you can put some thin batting in if you want it extra cushioned.
The measurements are from memory but of course you can alter them according to how long you want it to be and how wide your seat belt is.