RESPRO Waterproof Flo bag cover, £24.99,

The Waterproof Flo (or ‘Hump’) provides instant waterproofing and extra visibility for
your backpack. It’s well constructed, with taped seams, and an uncomplicated but 
effective pair of elasticated straps allows it to shroud rucksacks with up to 40-litre capacities. 
We used the cover in some torrential downpours, yet still found virtually zero water ingress.

The cover also includes an external mesh 
pocket for storing belongings you need quick 
access to, and large 3M reflective chevrons 
increase visibility at night.

Overall, this is a well executed solution. At first 
our reaction was to think it a bit pricey, but judging by the number of them we've seen this winter lots of people don't agree. On reflection, you realise it's less expensive than a backpack upgrade and might save a laptop from a destructive soaking. 

PROS waterproof, easy to fit, bright
CONS has ‘Hump’ written on it