Cubmaster126, Your mention of "vertical winglets added to the top and bottom of the elevator stab" would serve the same purpose as my suggesation of sub-rudders which would be located UNDER the fuselage, (sub = under).

Remember, I said IF you have stability problems with floats mounted, you could simply glue it on to the bottom of the fuselage at the aftmost location possible. You might GOOGLE the Japanese "Float Zero", the A6M2-N with its sub-rudder and then compare this with the ordinary Zero.

Don't forget, if you DO add a sub-rudder to see if it DOES make a difference on this particular model, you'll need to trim off some monocote so as to glue directly to wood.