If you searched you may have seen the one I put in Wren. I got the idea from someone else here and the name escapes me. So far mine has worked flawless. I do need to adjust the line since it has been wet and stretched a little. I was afraid mine might be to small but with it down it tracks like a train. I can still edge turn it but not very well.

Down it's triangular shaped and about 4" deep and 8" long. Of course on another boat it might not be enough, but I don't think it takes as much as more people think. Heck I don't even need mine, never use it except playing around.