How to make round Seitan sausages
The easiest way to make perfect sausage-shaped seitan would be to buy vegan sausage casing(assuming you are vegetarian or vegan since you are making your own seitan). 

Something similar to this.

Seitan in general isn't really smooth. And generally if it ever becomes smooth its probably over-mixed and becomes really tough. The casing provides a smooth surface and helps keep it in the round shape while you boil or steam it.