Very easy to do. I scratch built mine.
All you need is a few scraps of aluminum sheet (.080 or thicker) some tin snips a drill and a set of floats that you can mount the wheels in.
I copied full size retracts and built a little box for the main gear to go up inside the floats. Even had little gear doors that closed over the wheels for better water performance.
the nose gear retracted up to form the front of the float.
Retraction was done with a Futaba retract servo and a few feet of flex cable.
Sometimes I forgot to put the wheels down when I was land flying. Only forgot to put them up once when I was on the water.
Splash, trip.....sploosh!
I can give you more details if you are interested.
Peter