I took my two girls away on holiday with me as I just couldn't leave them behind at all. I personally think that a small dark carry case is best because then they can huddle up in the dark and feel secure. At first they were nervous, then they were fine with it. My carry case didn't have anywhere to clip the water bottle onto so I opened up the case and offered them a drink every so often. Soon they were talking away and eating food from my hands.
So basically yes it is fine to travel with your guinea pig. Bring fresh veggies with you and offer to your guinea pig later on, I doubt that for the first half hour or so she will want to eat or drink but she will become used to it.
Bring with you
fresh newspaper or towels
a small bag crammed full of hay
fresh veggies
water bottle
when you stop you can change the bedding and put it in a bag reserved for waste. You could even stop off to buy some fresh food.
I had a playpen which I set up when we stopped and the guinea pigs could get some fresh air and streatch their legs. Just don't leave her in the car by herself if it is really warm.
We often stopped to go for a walk along the way and I brought the carry case, we got photos of the guinea pigs at fox glacier, the tourists couldn't believe it, and I got plenty of comments "what you got there!"
I had never actually traveled with my guinea pigs before except to the vet but they were fine.
Good luck with the move
Oh and by the way, we traveled even longer then eight hours, but with stops at holiday houses, between the distances, for the night.