Dry bag/Rucksack
 

Just bought one of these that is basically a dry bag with a decent back/strap system.

Gourdon 30 l rucksack - Alpkit Shop

ive seen these before, ortleib do a very similar bag, but i cant remember the price, £20 seems pretty reasonable.
i use a large ortleib bag for kayaking and bike travel, its a good no frills sack that you can trust to be totally waterproof.

Bags.
 

Hi.
Yep, used the ort bags with straps before, decent kit!
Got about 10 in total (us kayakers eh)?
The alpkit one has a better back system and is more suited for carying for longer periods, well worth the 20 sheets!
Dave.