I have a bunch of dry bags in sizes from single lens or flash up to 30 litres, the soft ones are very light, fold up very small and are a good thing to have a couple off if the weather turns nasty.
Another kind you can get is a little more solid with a couple D rings on them and in the smaller sizes you can stash a second lens and wear it like a sling.
I get them from canoe shops.
www.dsaphoto.com
A picture is thousand words that takes less than a second while a thousand words is a picture that takes a month.