I've always used military flyers kit bags. They're olive green, pretty big, durable, and cheap. Plus, they fold flat.

The trick is to carry a few extra luggage straps (actually I use a set of strap clamps from the hardware store that I used to use when I was fixing some chairs). Anyways, you use those to cinch down the bag if it's too big. Two should be more than enough.

The other trick is that you can find cardboard boxes that fit inside it if you need it to be stiffer, when at your destination. Then your box has durable handles and doesn't look like a cheap box and has some added protection from getting torn up by baggage handlers.

You can find thse bags at surplus stores or all over the internet and ebay.