The thing that attracts most people to the flatback is its ability to run and float in skinny water. Adding a keel will increase draft. It may also interfere with water flow to the outboard requiring you to run the motor lower further increasing draft and decreasing efficiency.
I would be worried about engineering an add on keel that could withstand the stresses required to prevent skidding. Your glasswork would need to be flawless.
Honestly, your best option may be to find someone who wants to trade an Osprey for your Flatback. That way, you'd eliminate the skid and you would have a smoother ride.
