We bought a cordura-fabric top carrier for our mini-van. It is supposed to only be water-resistant, but in reality we have found it to be virtually water-proof. It holds a TON, because you can unzip it from both sides so you can really fill up every single corner.
Could probly fit all 4 kids in there in a pinch. (ha-ha, not really)
We got ours at Walmart for about $69. Best thing we ever bought for trips. At first I thought I'd be sorry we didn't get the hard-side type, but I've been very glad we got the one we did.
We never have to fight with getting it closed, or things stuck in the hinges, or worry if it really latched and locked.