Carsick kitty - any suggestions?

I travel with OmaKitty three times a year. Once to the vet, which is around the corner from our house, and twice to my parents' house for the holidays. Oma does fine going to the vet; it's not a long ride. But my parents live about 3 hours away and Oma gets terribly carsick, throwing up and pooping in her kennel after about the first 30 minutes in the car then when we hit a section of road about 30 minutes from my parents house that is in the hill country and we're going up and down hills, a bit like a rollercoaster.

I have tried all the vet prescribed sedation meds, Rescue Remedy, a few OTC meds that claimed to tackle carsickness... just about everything. At first, I thought it was an anxiety problem but after the last 8 years of riding in the car with her, I've determined it's not that since she only reacts when the roads get a bit bumpy.

Has anyone had any success with any medication other than sedatives or Rescue Remedy for travelling kitties? Due to Oma's "issues," I can't leave her behind with a petsitter or at a boarding facility - plus, she would just die if she found out I went to visit her Grandma without her.

I just realized I posted this in the wrong place... I apologize!

I"m afraid the only thing I've tried was valium from the vet. Would putting her in a completely enclosed box with no way to see out (except for breathing holes) work? Maybe its literally motion sickness. Maybe spraying feliway on some of your worn clothes and putting that in the carrier with her might help? Maybe the combination of feliway and your scent will sooth her, maybe in combination with a sedative from the vet. Gosh I can't even think of anything else.

I just purchased some Feliway and I'm going to try it out tomorrow on our trip to the vet's office for her annual visit. I just figure we'll take the long way there and maybe run some errands when we leave and I'll see how she does. But from what I understood on their website, it's more for anxiety than motion sickness so I'm not getting my hopes up.

She's the first cat I've ever had that didn't have problems in the car - all of my previous cats LOVED going places, so I'm a bit stumped on what to do with her.