Ollie, I'd make the tank a lot shorter and fatter and live with the frontal area penalty. Baffles will stop the fuel sloshing but it won't prevent it moving to the rear or the front and causing a large CoG shift. When it's full it won't matter but the dangerous time would be when it's 2/3 full and the model gets into an attitude that promotes the fuel moving to the front or rear. With DT's wish for a gentle handling model there won't be a lot of elevator authourity to combat such a state.

Someone a while back mentioned wing tanks. I think this would be a good thing. DT could put a spanwise tank in the wing that is about 2/3 of a gallon and it would just drain down into the fuselage tank that could then be a lot shorter. Keep the tank mounted just under the wing and gravity would handle the fuel delivery duties totally. This way the fuel is centered around the CoG much more tightly. I may be worrying about this for nothing but if it did turn out to be a problem then it would be a real pain once the model is built and being tested. I'm thinking the wing tanks and fuselage tanks would be independent but there would be a fuel line that would be hooked up as the model is being put together and then the tanks would be filled as one. There would need to be two lines actually. One for fuel and one vent to let air out of the lower and into the upper as they are being filled. The fuel line feeding down to the base of teh lower tank and the air vent coming off the top of the lower tank and up to the air space in the wing tank.