Looking at some wing theory material, you are essentially extending the original airfoil... Slats are the leading edge of that airfoil, so once you figure out the gap and AoA where you will be modifying the airfoil, you just have to build to maintain that shape. critical part would be attach points on the wings and the intern "ribs" in the slats to maintain the shape. I believe that hardest part is really keeping the shape once you finished.
To recap, the way I would go about it, take one of your wing ribs, now project that airfoil shape to extend beyond your current rib, find the AoA for best use and project that to the leading edge of the original rib, (some where in my notes I have 10-20% angle for the alpha but I did not take down the source, so no references for you now) so, assuming you get dual leading edges like ( ( your are creating an angle stating at the bottom of the leading edge like so ( > ( "well sort off" the trick is figuring our what % you are adding to the airfoil to get your shape.