I use large washable hospital-type underpads (bed pads) in the back of my cards where the kid's seats are. Thankfully it has been years since I worried about me having an accident on the car seats, but about a year ago I had an accident when I was out with no spare clothing and I just stole one of those pads for the driver's seat until I could get home. Worked great.
You can buy them online from the medical and bedwetting supply stores, but I tend to buy used, reclaimed ones on ebay, Often sold as puppy pads; sometimes they have brand new ones. I give them a round or two in a commercial laundromat with a sanitizer just to make sure they are properly sanitized.
I get years and years out of them when I use them as mess-catchers for the kids in the cars or under the food trays when I let them eat on the carpet. Or we use them when one of us needs a procedure done at home.
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