Haven't seen anyone suggest this yet, so--
I recently had a little cash windfall, and splurged on one of those 2.25-inch-thick bamboo Go boards and yunzi stones. Looking for a good way to carry/store the board, I read reviews complaining that the board bags offered by YMI only fit very specific dimensions and were thin cloth that offered little protection.
So I looked for an alternative and found a good one.
Pizza delivery bags!
They are the right size and shape. The one I got ("Holds THREE 16" or TWO 18" Pizzas") has enough extra room to tuck in additional padding and a book or two, or maybe even a second board.
They are water resistant.
They are insulated with padding.
They are even available in different colors and styles.
About the only disadvantage to them is that the handle is placed awkwardly (they expect you to carry the pizza upright, after all.) And they're slightly pricey if you have to pay shipping. I paid about $20 on Ebay.
http://www.ebay.com/sch/Pizza-Delivery- ... p=15&_sc=1
Still, if you have a big board (and especially if it's an expensive one) and want a safe way the schlep it around, this seems like the best solution out there.