You can get bigger plastic crates (aimed at dogs) with castor type wheels. I've got one that I can probably get 3 or 4 cats in - although I coudn't lift it on my own at that point, and it won't go through a standard car door (just in the boot). It's a rigid, Pet Voyager / Ferplast Atlas type carrier that comes as a 2 part rigid shell and separate clip on wheels.
I have used small standard dog crates to transport multiple furries to vet appointments - but again, you still have to check that you can get them in the car. There are generic clip on handles that make them easier to carry (better with 2 people).