LOL, I'm in the same boat, but I trust that our very short journeys (10-15 minutes at the MOST) & going potty before we leave will prevent any leaks until dd is more reliable.
My best suggestion (and one that I did when she was having diarrhea blowout diapers), is to look in the baby section of stores for something called Lap protectors or along that line. They're various sizes of thin cloth imbedded into plastic. Extremely maliable & waterproof, especially after laundering the first time, but are very thin and solid. They don't even have the thickness of a T-shirt and with no compression at all.
The ones I used were no-brand AAFES. 3 sizes were
small -5X5"- for a lap --
middle (perhaps 12X16), for an area across a crib,---
Large which was designed to go under a fitted sheet to protect an entire crib mattress.
The small was a bit too wee to protect the seat & sides, so I'd take scissors to the medium size which cost somewhere around $2.50. There was absolutely no adjustment needed for her carseat harness, but the seat never got splashed with sewer baby bottom during a 10 hour drive (with breaks of course). Sad to say that she went through 4 outfits during it though. Poor baby
I'll be using them again when we go on some long road trips in the next few months just in case.
I've heard about the 'piddle pad', but it looks too small, too bulky, and costs a lot more money than the cheap crib protectors