I agree with Jim on this.

After you make the rails why not mount them temporarily with a few drops of hot-melt glue and then go paddle for a day or three. If you get any knuckle-knocking problems on the shakedown trials you'll know more about where to move the rails for a permanent installation with epoxy.

Also check to see if the location of the rails causes any water to spray onto you, or if instead it deflects water aside.

PGJ