A large heavily waterproofed nylon sack. A cheap way to make your rucksack reliably showerproof.
On the bag you can find detailed instructions on how to tie the mouth. There are simpler ways too. Dimensions about 55 x 40 cm, at least in the style we packed it.
Used, might occasionally have small holes here and there, mostly some pinhead size, nothing major. However these do work well as packsacks, just don't go diving with it. These do have a rubbery smell to them and the bag will never truly wear out, be warned.
Adrien D.