Frank, if your dekes are narrow and high, it may be necessary to work with a taller, narrower. keel. 
Steve covered the flat, wide keel on Al's stuff pretty accurately-On a low water situation, the decoys would still sit flat, as opposed to laying on their edges. 
A deeper keel will probably solve the tall and narrow problem, BUT, if the tide goes out, they  will not look reasonably realistic,lying on their sides! 
The alternative is to hang a swing weight on your existing keel. Down side on them is if you are dealing with a number of decoys-Chance of causing scratches and damage increases/ 
It's all about trade-offs. isn't it?