Which cage should I buy?

Use guineapigcages.com for cage size info, but you should really think about getting two piggies(they like to live in pairs or groups). And yes it is easy to put a cover on it.

You can build your own if you can find all the supplies. I found coroplast for $10 from this place. (broken link removed)

Grids from: (broken link removed)
With 20% off coupon, it's only $12.99 per box. And you'll only need one if you're not building a stand.

Follow instructions on this page for how to build.
How to Make a C&C Cage

Please adopt a friend for your pig. (don't get one from petstore or breeders)

You can make lids out of grids or closet shelving. If you have an animal that may jump on the lid (like a cat), I'd suggest using the shelving, as it's much sturdier and less likely to collapse under the weight.

You should try to make the C&C cage as large as you can afford with your materials.

My questions is why can't you MAKE a C&C cage? It's really simple, I did it in less than 15 minutes. And it cost me only $30 for the whole thing. It's a 2x7, so yours will probably be cheaper if you go smaller.

For one guinea pig, your C&C cage should be at least a 2x3. It is true that guinea pigs love space, and I have two guinea pigs who live in a 2x7 and they love it. It also seems like they need more space with all their toys, but for now they are happy.

I strongly encourage you to adopt a second guinea pig. As social pets, guinea pigs love the companionship of another of their kind. I used to have just one guinea pig and once I saw how lonely she was, I immediately went to get another one. You REALLY should adopt another one. If you do, you'll need to make your C&C cage AT LEAST a 2x4. But again, you should really make it as large as you can.

I bought my grids from this woman on craigslist. You should try there, as a lot of people sell their grids for cheap.
I got my coroplast from the GE Polymershapes place that lissie said above. I bought my fleece from Joann Fabrics, 2.5 yards for only $15. Mine is a 2x7 and I have two sets of fleece for when I'm washing the other. I'm really happy with my C&C cage. (And so are my pigs!)

Lids are said to be pretty easy to make, but I wouldn't really know for sure as I don't have a lid. I have a terrier who has a pretty big prey drive, and he hasn't tried to jump in [yet]. I think all you do is connect the grids over the top. But I'm not completely sure.

Good luck!