I have over 3,000 miles on my litter boxes. They are cheap, waterproof, durable, double as camp chair and table, and weigh the same as ortlieb back-rollers. A purpose designed mounting system (like ortlieb's) might be better than what you come up with, depending on how clever you are, however, I have never had a problem with my mounts, even on some rough trails and washed out dirt roads.