I would look at just re-purposing some motorcycle luggage for it. If you buy off brand you can usually find reasonable prices and most of it is waterproof as well. I usually buy all of my motorcycle stuff used and if you shop around, you can get stuff really cheap.
Bike Bag Shop has more bags than any other place I know of. I particularly like the trunk bags where they click into place and remove by the push of one button. They carry the best, most expensive to the modest, more affordable.