I think I might have answered this one myself (eventually) after seeing above ideas I was walking around Dixons of all places looking for that sale 'bargain' when I stumbled on the Miranda Tripod Bag (to fit model TP35) - its an absolutely perfect fit for a 3ltr bottle with room for some padding (foam or bubble wrap etc). This bag is strong nylon with waterproofing on the inside, two strong webbing straps and a zip top - ideal for keeping dust out of your valve and hose - the best bit is they are just £4.99 !!! Couldn't believe my luck and plenty in stock. Worth a try at that money I'd say !!