RE: seamaster .40 rudder question
Stretch an elastic band from one side to the other under the fuselage hooked around the gear legs. That should prevent them from falling out.
As to the water rudder question, tin is sharp and could cut your finger and plywood falls apart after it has been wet for a while. A piece of plastic sheet will last the longest, bend over when you put the plane down on the water rudder and last forever.
Peter