The banana boxes I've seen aren't all that water proof. According to the dimensions I found on the internet they are 19 in x 12.5 in x 8.25 inches for volume of 1959 cubic inches or 8.5 gallons.
If I was going to use boxes I'd try to get those that produce like lettuce comes in. They are much more heavily waxed and might stand up to being wer inside and out.
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