These guys will make a custom cover for 30 dollars plus shipping out of this material.
http://www.mytarp.com/vinyl-tarps-10-oz-coated.aspx
I think a good cover will be important for the welder in a shop like mine. My Lincoln has lasted 8 years but all of its electronics are encased in what looks like solid silicone. I don't know if that is industry standard, but it seems like a good idea. The pics I've seen of the 250ex have the electronics open.
I'm going to order the cover @ 24x10x17. It's a little oversized as I don't want to struggle over knobs and hoses every time I put it on. Now the only question is green or black.