We have a Purple Line Stowaway bag. Have used it 3 times, once it really wet weather, and it didnt leak at all. Folds up nicely into its own bag and is a great option if you dont have room for a box.
I have the one in the ebay link, good for the price, although the base is one piece, rather than stips like some of the others, well made, and seems to be 100% water proof if you close it properly, driven 150 miles in siling rain, and everything inside bone dry.