I bought a Doofer last year but I have already used it for my own s/s conversion.
PS If you do buy a Doofer, you may find this useful ... I found that the chain came off a couple of times because a Doofer normally pulls the chain away from the sprocket. I fiddled with the spring and got the device to push the chain up instead which means the chain wraps round several extra teeth. It is very reliable this way round. (I don't know why it wasn't designed that way in the first place!)
The chain was new and tight in that picture. It has stretched a bit since then and is being pushed up more now.