IF you could gaurantee that the airflow would always be inline with the square corners then there would be no advantage to using rounded corners. This would mean that you would have to know exactly what the air is doing at all points as it slides down the length of the fuselage. Since most of us don't know how to figure out the airflow to that degree it is unavoidable that some of the air is going to hit some of those corners at an angle. As it flows around that angle it creates turbulence and drag. Parasitic drag.

But a nicely rounded fuselage will go a long way to reducing the size of any vortices. The rounder the better. And since most of the angular airflow is probably in the vertical plain it usually is more benificial to make the fuselage in a vertically oriented oval shape.

How you do that is up to you but the "classic" method is light wood used for thick sides, even thicker tops and triangle stock in the internal corners. From there you carve and sand it all down so that it is as rounded as practical given the need for structural integrity.

Keeping the frontal area down as much as practical helps a lot too. That means the fuselage needs to be skinny. Or at least as skinny as possible other than the need for those blocks to help shape the roundness. The best of both worlds is a fiberglass fuselage that would be just big enough to hold the engine, tank and gear. Better yet is a high output engine with a rear exhaust venting into a tuned pipe or mousse can muffler in a ducted shape in the fuselage directly behind the engine. All else being equal frontal area is the enemy.

In the end you can't do much better than using some of the electric Speed 400 racer models as a pattern. This shape is often seen in the British Club 20 pylon class as well. You don't see it used in much of our other North American racing classes because of rules governing cross section area, shape and engine/exhaust system selection.

An engine cowl is a good idea but if you have a rear exhaust or a manifold that folds the exhaust around to the rear then a muffler shroud "bump" behind the engine cylinder can offer a lot of assistance in dampening out the turbulence and aiding in going faster.

But all this fades by comparison to the need for an engine that can spin a higher than normal pitched prop as high rpms. If you don't have the power it ain't going fast. That's why a "dirty" planform like the deltas with exposed servos you see in the high speed prop forum can do so well. They often put a fancy engine on the front that just drags the rest along in spite of the protests....

.... so much for aerodynamic finness...