I also have a few Lastolite softboxes (Ezyboxes and a Hot Rod Stripbox). The Ezyboxes easy to set up (I guess that's why they're called that
), but my 24" ones are pretty big when collapsed. The 15" and 18" versions are more portable.
The Lastolite products aren't what I'd call cheap. But if you'll be using a softbox a lot and carrying it in a backpack, you'll probably want something pretty durable. With a cheap softbox (Cowboy Studio, etc.), I'd worry about the stays getting bent, etc.
With an off-camera soft box, you'll also need to consider a way to mount it, such as a light stand, and some way to trigger it (preferably via radio – Canon's optical triggering isn't very reliable, since the optical signal is often blocked by the softbox itself). A heavy-duty Gorillapod might work to support a smaller/lighter softbox attached to a chair of a pole.