I'd consider a rubber sleeve - one of those which SS hose clamps at either end. You might be able to stretch it to fit one end, and clamp it enough to seal at the other (depends on what's available which end might be bigger/smaller). What provisions do you have once the barrel is full? You certainly don't want it backing up in the downspout! Most of those I've seen have an overflow outlet. They are arranged so that you can daisy-chain multiple tanks together to expand your storage capacity.