Snugpack 35L rucksack cover.
Useful item this, and not just for Snugpack backpacks, but most others too, (35L and below). This is a tough ripstop nylon waterproof cover that quickly and easily depoys over the top of the backpack/daypack to provide and barrrier to the elelmets. Stops water being driven into your pack and soaking clothes and food and kit. Crushes down small and comes with a draw string bag. Stuff it in your backpack and use it when ever the weather threatens!