The answer is probably really obvious, but I'll ask anyway.

You know those big, flat bags that artists have to carry their work around in? They tend to be large enough to lay a big piece of paper or canvas flat inside of it, might have a few pockets on the outside, but usually aren't much more complicated.

Do those things have an actual name beyond "big flat artist bag"? I'm fairly sure they do, but everything I come up with doesn't seem quite right.