Maybe you can find downspout material that matches the size of your downspout that enters you rain barrel. Drill a hole with a hole saw and then tap a downspout inlet into it. Then attach downspout material to the inlet. It really helps to have downspout crimpers to piece downspouts together. But in a pinch one might use pliers. Pretty tough to do though without the proper tools.

I'm just starting up with rain barrels, but have done a fair amount of research on the subject. Even though the diameter of the downspout leading into the container is larger than that of the overflow valve, that shouldn't be a problem. In my barrels, the interior part of the valve is located about a foot below the top of the barrel, so water would start exiting the valve long before it can top off. If you really expect your roof to give off that much water, think about getting more rain containers; either positioned at other downspouts, or connected to the first rain barrel via the overflow valve. If your barrel has a spigot, you could also connect a garden hose that you have leading away from your foundation.

Just buy the kit from Aquabarrel . com Their design is based on a balanced system of equal volume in = volume out ;-)

Here is a link that might be useful: Aquabarrel Parts Kits

I have three 55 gallon barrels connected together .We cut the downspout off above the barrel by about a foot. Then
attached a flexible spout fom the home improvement store to it and used a small screw to hold it on. Then we attached the cut piece of downspout to the house about level with the barrel. Then we ran a spout down away from the barrel. If I want to direct the flow out of the rain barrel, I put the flexible spout in the down spout and the water runs away from the house. If I want to catch water, i allow the flexible spout to hang on top of the first barrel. When that barrel is full the water flows into the second and third barrels via a pvc pipe inserted thru the barrel near the top.
What we did was to drill a hole 3 or more inches from the top and place pvc pipe to connect the barrels.On the last barrel , we put apvc elbow in the side about three inches from the top and ran a piece of pvc down to almost ground level and then put another elbow and then attached another piece of pvc . We used silicone caulk to seal the pvc in place on the barrels. This way all three barrels will fill, but your over flow , flows away from the house.

Hope this helps.

I just got an e-mail from http://www.RainTankDepot.com 's mailing list that is an Earth Day sale on 4/22 on Rain Barrels and Composters. Use code 'EARTHDAY' for 5% off.

Here is a link that might be useful: RainTankDepot