Definitely an A1 artist portfolio if you live outside of North America and such things are easily available. For those who live in North America, 24"x36" portfolios are more readily available and are slightly larger than A1 so the A1 maps will fit nicely.
Most artist portfolios are soft-sided, which makes for cheap shipping but poor protection. I also bought two sheets of coroplast (corrugated plastic) in the same size as my portfolio (cardboard would also work). My maps are sandwiched between the two sheets of coroplast, and enclosed by the portfolio. My custom Canada map and Fortress Challenge, both printed on neoprene, are stored on the outsides of the coroplast sandwich, just to keep all the Carcassonne stuff together.