[pigsforlife]

I would suggest a dog or cat carrier with hard plastic sides. Cardboard boxes are not very safe, especially if you were in an accident plus it has the opportunity to make a huge sodden mess if they pee on it etc. I have a dog carrier for a medium sized dog that provides ample room for the short trip to and from the vet (20 minutes either way). Having access points from the top and front is helpful but not essential. My carrier has a wire front that I use to attach a water bottle (be careful though, depending on your water bottle it can make a huge mess). I always chuck in mound of hay or wet veggies to keep them happy on the way as well. As you probably know, guinea pigs need to eat all of the time so I use hay/veggies to get around car journeys. Of course, if you were going for a longer trip you would have to bring their normal food (eg pellets, hay 24/7, veggies twice a day) as well but for the sake of a vet trip just hay is fine.

So I would recommend;
- A hard sided cat or dog carrier
- Line the bottom with something, I usually use towels
- A mound of hay/veggies
- Keep the temperature comfortable
- Keep noises to a minimum (eg no loud music)